A slow moving storm system will bring a prolonged period of unsettled weather to the NYC region this weekend. The upside of this storm is that it will usher in some warmer air from the south, giving us some above average temperatures for a brief spell. Behind this storm system a cold front is forecast to pass through early next week and return us to below normal cold for this time of year. I will be following up on this post with a detailed forecast for Saturday, as I attempt to keep my forecasting skills sharp.
Rest of today – overcast, with high temperatures around 50ºF. Increasing chances for rain going into this evening and overnight. Low temperatures in the mid-40s overnight.
Surface forecast for Saturday morning, 7AM
Saturday – similar day to today with a generally gloomy feel, overcast skies with a chance for rain, though looks like precipitation will be on the light side. High temperatures in the low-50s. Overnight lows into Sunday cooler than Friday, in the low-40s.
Sunday – yet more rain possible with the storm slowly making its way east and transitioning to a coastal storm. High temperatures are expected to be cooler because winds will shift onshore from the northeast – highs probably in the mid-40s. Overnight into Monday, temperatures should hold in the upper-30s with rain chances winding down.
Surface forecast for Sunday at 7AM
Monday – skies slowly clearing with high temperatures in the mid-40s.
I haven’t had time to post about the current week’s WxChallenge forecasts for Caribou, ME, which I’ll have a follow up post about Saturday or Sunday to go over my results. Instead, this week, I’m going to put the same forecast skills I’ve been applying to the upcoming nor’easter that’s going to impact NYC Saturday. I’ll provide a deterministic (exact number) forecast for high temperature, low temperature, maximum sustained wind speed, and total accumulated precipitation for the period starting 2AM Saturday and ending 2AM Sunday, with a less exact forecast for the rest of the weekend. In my later follow up post, I’ll verify my results and see just how close I got. For purposes of verification, my forecast will be for LGA. If you’re only reading this summary, just know that Saturday will be a windy, raw, rainy day, but that conditions will improve as the day goes on.
Friday – partly sunny skies with high temperatures in the low-50s. Light winds from the south-southeast.
Saturday – High temperature: 54°F. Low: 48°F. Maximum sustained winds (2-minute averaged): 34.5 mph from the east-northeast. Total precipitation: 1.25″. The bulk of the rain should end by the afternoon hours.
Sunday – mostly cloudy, high temperatures in the upper-50s with return flow from the west downsloping into the area.
Forecast Discussion
The nor’easter that’s coming to us this weekend has tropical origins as the remnants of Hurricane Willa, which originated in the Eastern Pacific and made landfall in Mexico as an intense Category 3 storm. This means that it will be able to tap to tropical moisture, bringing the potential for a heavy rainfall event. At the surface, a low pressure center will be intensifying as it slides up along the Mid-Atlantic coast. At the mid and upper levels of the atmosphere, the set up is favorable for robust divergence, which should result in to strong surface convergence and lift. At the 850 mb level, there are indications of a strong low-level jet with winds as high as 50 knots from the east to east-northeast during the Saturday morning hours ahead of the warm front attached to the nor’easter. This would enable highly efficient moisture transport and convergence in our area. The net result of this should be periods of heavy rain. This is what justifies rainfall totals over an inch, though because of the relatively fast forward speed of this storm, I don’t believe that totals above 1.50″ are likely.
These aren’t pictured below, but MOS (model output statistics), and forecast plumes (ensembles of many different iterations of the same forecast model) probabilities suggest a consensus range of 1.15″-1.24″. Short Range Ensemble Forecast (SREF) probabilities show a greater than 90% chance for rainfall totals greater than or equal to 0.50″, 70-90% chance of totals greater than or equal to 1.00″, but very low chances of totals exceed 2.00″. The record rainfall for Saturday at LGA is 1.76″, though there are higher record totals surrounding this date, so this record isn’t what we’d call particularly strong.
On the temperature front, the trajectory of the onshore flow accompanying this storm actually brings it over Atlantic waters that are warmer than the air temperatures have been this week. This should bring warmer air into the area, however, because it will also be raining and cloudy, evaporational cooling will be a concern (as it rains, some of the moisture from raindrops will evaporate, which takes heat input to achieve, lowering surrounding temperatures), and that will likely knock temperatures down a bit. Overnight lows should end up being fairly mild in comparison to previous nights because of cloud cover and the aforementioned winds over warmer waters.
The wind speeds should be quite strong tomorrow. MOS is showing winds in the 24-26 knot range. However, given that 850 mb winds will be even strong, in the 50 knot range, and it will be precipitating, meaning downward transfer of momentum by raindrops should mix some of these stronger winds to the surface, which is why I think sustained winds could be as high as 35 mph at some point in the day at LGA (which is exposed near open water, so there’s less of an impact from friction over the land). I think gusts could easily top 40-45 mph tomorrow during the most intense rain squalls.
Surface forecast from the Weather Prediction Center valid for 8AM Saturday, showing the nor’easter in question
850 mb forecast for relative humidity and wind valid 8AM Saturday. The dark blues show saturated air mass, and the wind barbs show strong onshore winds
500 mb vorticity and heights, the yellow-orange-purple shading shows progressively stronger values of positive vorticity. Without getting too technical, positive vorticity is related to divergence aloft, which helps aid upwards motion (lift). Strong upwards motion can lead to clouds and precipitation.
Shaded areas indicate strong winds at the 300 mb level. These are referred to as jet streaks. Our region is forecast to lie within the left exit region of a jet streak, an area that favors divergence and enhanced upwards motion/lift
Not surprisingly, the SREF probability forecast shows a 70-90% probability of omega (a quantitative measure of lift) exceeding -9 microbars/sec, which is considered quite strong
Update: Verification
According to the KLGA Daily Climatological Report showed the following for Saturday, October 27, 2018:
High Temperature: 53°F
Low Temperature: 45°F
Max Wind: 41 mph
Max Gust: 50 mph
Total QPF (this is taken from METAR data): 0.98″
I’ll attempt to do a post-mortem on the winds and QPF forecast where I missed pretty significantly later in the week.
A couple days into the forecast period for the second WxChallenge city for this fall, and I’ve experienced my worst forecast bust (for yesterday). Highlights in the post-mortem section. It has been a week of transitions for Omaha, which started off with relatively mild conditions but will end the week with possibility for patchy frost!
Synoptic Set Up
A 500 mb longwave trough will with multiple attendant vorticity maxima will pivot through the KOMA region during the first half of the forecast period before flow becomes more zonal. At the 300 mb level, multiple jet streaks could boost divergence with these 500 mb vorticity maxima. This enhanced lift, and strong wind shear profile could lead to some isolated strong-severe thunderstorms. The 850 mb shows evidence of a decently robust LLJ that would should bring a continuous supply of Gulf moisture up the Missouri Valley into the area. At the surface, a warm front looks to lift through Monday, then stall as a cold front in the vicinity of KOMA through late Wednesday due to the deep unidirectional winds parallel to the front boundary.
Notes
The potential for flash flooding, and bouts of heavy rain, is duly noted in the AFD during the first half of the weak. The synoptic ingredients for a long duration rainfall event are certainly evident. Soils in the area are already quite moist, given that flood warnings were present for stretches of the Missouri last week. GFS MOS QPF range on Sunday for 06Z Tuesday to 06Z Wednesday was 1.75-4.46”. NAM MOS QPF range for this time was 1.35-2.71”, consensus was 1.55-3.59”. Records around this time of year for QPF show that 1-2” can easily occur, despite the normal being only a mere 0.07”. SREF probabilities of >= 0.50” were above 90% for this period, and >= 1.00” was 50-70%, but over 2.00” of QPF probabilities were less than 10%. SREF mean was 2.73”, with a pretty large spread, from 0.69-5.54”. GFS mean was 3.93” with spread of 1.82-5.16”. For now, going with a superblend of the low-end of the ranges of MOS and plumes: 1.40”.
As of Monday, SREF and GEFs means had become much better aligned, with SREF mean at 2.83” and GEFs mean 2.95”. SREF spread was 1.34-4.46”, GEFs was 1.43-4.43”. QPF forecast shows KOMA a pretty large QPF gradient, and that uncertainty seems borne out well in the ensemble forecasts. SREF probabilities of >= 2.00” increased to the 10-30% with KOMA lying right on the fringe of 30-50% probabilities of this amount. >= 1.00” probabilities increased to 70-90%. Official NWS forecast as of 10Z Monday called for 1.65” during the forecast period Tuesday. GFS MOS QPF range was 2.10-4.21”, NAM 1.01-2.06”, consensus here is 1.55-3.13”. Based on these updates, I’m increasing my forecast to 1.85” – I’m hedging down below the SREF and GEFS means because I’m still not sure where the stationary/slow-moving surface front will set up in relation to KOMA, and where the best lift will be for heavy rain/thunderstorms as a result. Also, if the 850 mb LLJ fails to materialize close to KOMA, we could be looking at lower overall totals. Low SREF probabilities of over 2.00” also help justify this call. Looking at Wednesday, rain chances diminish considerably after the front moves through, but the story then becomes strong winds possible, with MOS pointing at 17-20 knot synoptic winds.
Ensembles continue to trend upwards as of Monday afternoon with QPF, SREF mean now 2.96” and GEFS mean is 3.16”. SREF probabilities are close to placing KOMA in the 50% >= 2.00” QPF. Will take one more look at 18Z MOS guidance and some other factors before finalizing, but am leaning towards increasing the precip forecast again, this time to maybe 1.90” I’m wary of going for 3” of QPF or higher, given that the entire month’s single-day QPF record is 3.09”. Even beyond 2” still seems unlikely.
On Tuesday, looking ahead at Wednesday and Thursday, the passage of a cold front is quite evident. Temperatures are forecast to drop considerably compared with the beginning of the weak, and precipitation will end, too. The forecast for Wednesday still retains a slight chance for some measurable QPF on the tail end of the main precipitation event Tuesday. Models have backed down from up to 0.25” of precip to less than 0.10”. SREF and GEFS means show approximately 0.04” on Wednesday. SREF probabilities showed that KOMA had a greater than 90% chance of receiving precipitation >= 0.01”, but less than 10% of >= 0.10”, going to go straight up with the ensemble means here. The other story for Wednesday will be stiff winds from the northwest, with MOS synoptic winds of 17-18 knots. This suggests even stronger sustained winds, and will aid in cold advection, keeping high temperatures suppressed. These blustery conditions are forecast to gradually abate by late in the day Thursday, such that the strongest winds Thursday should occur early in the overnight hours. Cold temperatures overnight into Thursday should be somewhat tempered by decent vertical mixing and overcast skies.
Forecasts Submitted
This only reflects finalized forecasts, I’ve made small adjustments to the forecast based on checking MOS daily.
Forecast Day
Max Temp (°F)
Min Temp (°F)
Max Sustained Wind (kts)
Precipitation (inches)
10/09/2018
62
57
14
1.91
10/10/2018
48
44
20
0.02
10/11/2018
48
34
17
0.00
10/12/2018
48
35
9
0.10
Post-Mortem
Day 1: Major bust across the board. Was off by more 5 degrees or more on both the high and low. This made up about half of the error points. The rest came from being off on max precipitation by more than 1.20”. MOS was considerably off the mark with temperatures – though NAM was closer. This was all due to the fact that a stationary frontal boundary never made it close enough to KOMA to realize warmer temperatures, and heavier precip. The axis of the 850 mb LLJ stayed south and east, along with the best surface convergence. As a result, nearly all of the model guidance was horribly wrong. The closest forecast in the class was still nearly 0.50” too high. I put too much credence into the upwards trends on the models – there were some indications already that KOMA was trending to be on the drier side of this event. Still, even if I had nailed the precipitation forecast, I would have still busted hard on the temperatures. Need to be a lot more cautious going forward about situations where a frontal boundary and precipitation are around, and probably hedge down on temperatures during transitional season when a forecast site could be on the cold side of a front with rain.
After the first week of the WxChallenge (and part of this week), I’m second place in my class, tied with multiple others for #268 nationwide, and a little better than national consensus with 21.2 error points. I got hit the last day for going over on temperatures and winds, and will discuss that below. Hats off to one of my classmates who’s currently #44 nationwide!
Verifications
Below are the verified readings for each metric we were asked to forecast during last week, and a column showing error points I was assessed.
Forecast Day
Max Temp (°F)
Min Temp (°F)
Max Sustained Wind (kts)
Precipitation (inches)
Error Points Assessed
09/25/2018
82
50
11
0.00
4.0
09/26/2018
83
52
11
0.00
1.0
09/27/2018
84
52
9
0.00
2.5
09/28/2018
86
53
11
0.00
6.5
Post-Mortem
Day 1: went too aggressive on the max winds, and too warm on the low temps. The lower winds could explain the lower min temps (less mixing on clear nights). I think I was too optimistic about the possibility of funneling of winds from the ENE along the Columbia River Gorge.
Day 2: did pretty well, only got 1 point off for missing the high temp slightly. If I had stayed with my original forecast for that I would have had a perfect score! Much better on winds though.
Day 3 & Day 4: another decent day, but went a little too cool with the low temperatures. This low bias also bled into Day 4, where I was low on both the high, low, and max sustained wind speeds. I think if I were to go back and redo this, maybe I would have used the Delta Method (a form of moderate persistence) and been better off than looking at MOS, given how stable conditions were throughout the week.
Forecasting for KPDX October 2-5, 2018
Synoptic Set Up
Big changes in store for Oregon in the second week of the forecast challenge. The rex block and high pressure that brought a week of above normal warmth and clear skies last week will finally break down. The upper low associated with it will move northeast towards KPDX over the weekend. Local NWS forecasters currently (as of Thursday) noted considerable spread in forecast guidance about phasing of northern stream energy and a second upper low by the beginning of this forecast period, as well as interaction with outflow from Hurricane Rosa. AFD mentions longwave troughing as well, and this is backed up by GFS progs.
Rainy start to the week at KPDX possible
Coastal low coming ashore on Friday at KPDX
Forecast Rationale
The effects of the synoptic set up favor much cooler and unsettled conditions compared to last week. A drop in 500 mb heights with upper lows and longwave trough should result in temperatures close to or slightly below climatological norms. The prospect of this troughing means that precipitation will be in the mix as well at the start of the week. More substantial rain chances are in store for KPDX to end the week. Along with the rain Friday, we can expect cooler temperatures and windy weather as rain cooled air helps mix down some momentum from the lower atmosphere.
Forecasts Submitted
This only reflects finalized forecasts as of this posting, I’ve made small adjustments to the forecast based on checking MOS daily.
Severe thunderstorms could impact the area later today ahead of an approaching cold front. Following the passage of this cold front, conditions will begin to improve, setting us up for a weekend with pleasant, cooler, classic autumn weather.
Rest of today – Storm Prediction Center in Norman, OK has placed parts of the NYC Tri-State (the city, north and west) under a slight risk of severe weather today. Warm, moist air (read: unstable, theta-e rich) is in place with the area in the warm sector of an advancing low pressure center located in Ontario/Quebec. Some rain/thunderstorm activity could take place along a weak warm front this morning. The main focus of activity will occur later today with a cold front approaching from the west.
This cold front will be accompanied by a streak of strong upper level winds (40-50 knots), which will be capable of providing the magnitude of vertical wind shear necessary for some strong to severe thunderstorms to organize. Given the direction of the winds at upper levels being largely parallel to the advancing frontal boundary, expect that the primary storm format will be a QLCS (quasi-lineary convective system) – a line or broken line of storms. High Resolution Rapid Refresh models depict the bulk of storm activity approaching the city around 8PM this evening. Given this storm format and synoptic set up, the most likely severe threat will be damaging wind gusts. Heavy rain could trigger flash flooding as well, since the ground is still quite saturated from yesterday’s rain.
The biggest limiting factor to storm strength today will be overcast skies limiting daytime destabilization from solar heating. As is often the case, I would expect the storms to be weakening somewhat as they approach a more stable marine air layer near NYC. Overall, expect a cloudy day with high temperatures in the low-80s and a muggy feel.
Storm Prediction Center Day 1 Convective Outlook
Weather Prediction Center forecast surface conditions at 8PM EDT
High Resolution Rapid Refresh simulated 1-km radar for 8PM EDT
NAM forecast for 500 mb winds and height valid 8PM EDT
Thursday – the cold front that brings us this possibly severe today will slow down as it pushes south of us, such that it remains close enough that a weak disturbance moving along it could bring us some additional showers late in the day. Skies will be partly sunny with much cooler high temperatures in the upper-60s.
Friday – easterly onshore flow on the back side of this cold front forecast to be to our south will keep skies cloudy with high temperatures around 70°F – chance for showers lingers.
Saturday – high pressure finally starts to take control of sensible weather and should produce a mostly sunny day with crisp conditions and high temperatures in the low-70s.
Cooler weather arrives just in time for the astronomical start of autumn. Cloudy conditions have been persistent, and will continue today. A chance for rain and scattered thunderstorms tonight going into Saturday from a passing cold front. These will not be drenching rains like with the remnants of Florence. High pressure returns Saturday, and should give us a dry weekend, though we may continue to see more clouds than sun.
Rest of today – cool, and cloudy, with high temperatures in the low-70s. Chance for showers and thunderstorms overnight with a passing cold front.
Saturday – sunnier with high temperatures in the mid-70s. The autumnal equinox will take place at 9:54PM EDT.
Sunday – partly sunny with high temperatures in the low-70s.
Monday (Mid-Autumn Festival) – partly sunny with high temperatures again in the low-70s.
We had some great late-summer weather this past weekend with high temperatures in the low-80s and lots of sun. Most of this week will be quite nice as well, with the notable exception of Tuesday, when remnants of Florence impact the region and bring around a day of heavy rain. Fortunately, this wet weather will be short-lived, and the remainder of the week should see a return to mostly sunny conditions.
Rest of today – increasing clouds and humidity as remnants of Florence approach from the south. High temperature around 80°F.
Tuesday – periods of rain, heavy at time, with scattered thunderstorms possible. Otherwise cloudy, with high temperatures around 80°F again.
Wednesday – conditions improve as the remnants of Florence quicly move off to the east. A return to mostly sunny weather with cooler highs in the upper-70s as the back side of the cyclonic rotation of the remnants of Florence brings a cold front through.
Thursday – another mostly sunny day with high temperatures in the low-70s, high pressure building should continue to keep things dry for the region.
After a week of gloomy, cloudy, and at times rainy weather, we will get a nice break this weekend as conditions improve. The same high pressure that’s caused Hurricane Florence to make landfall on the Carolinas by blocking its northward progress will bring us sunnier weather. Indeed, some of the cloud cover over our area is actually a result of outflow from Florence. Its remnants will impact us with rain at some point early next week.
Rest of today – mostly cloudy with high temperatures in the mid-70s as winds continue from the east. High surf with waves 8-12 feet are possible at the coast due to Florence.
Saturday – decreasing clouds as high pressure continues to build. High temperatures around 80ºF.
Sunday – sunny, high near 80ºF, high pressure remains in control.
Monday – clouds and the chance for rain returns. More moist air returns as the high pressure that gives us nice weather over the weekend slides east (and the flow on its western side starts to bring moisture from the south up). We’ll need to monitor the progress of the remnants of Florence as they are likely to bring us rain.
This week kicks off with a prolonged period of unsettled weather as a warm front and a cold front both impact the area. Chances for rain will be with us through Wednesday. Conditions finally improve late in the week. We luckily will avoid direct impacts (aside from high surf) from what will be an extremely dangerous Hurricane Florence, which looks set to make landfall on North Carolina late this week. Aside from Florence, two other storms pose threats to US territories this week.
Rest of today – cloudy, with periods of rain, heavy at times, especially later in the day towards the overnight hours. High temperatures will be in the upper-60s with cool, persistent onshore flow from the east. Ahead of an approaching warm front.
Tuesday – chances for thunderstorms will persist into Tuesday, with this slow-moving warm front finally pushing through. There could be thunderstorms overnight. Then, later in the day, as the trailing cold front moves through. Once the warm front passes over, we’ll be within the warm sector of the parent low pressure system, which will allow temperatures to climb into the low-80s with a marked increase in humidity.
Wednesday – the cold front mentioned above will slow down as it approaches us and dissipate. This will lead to continuing chances for rain, and thunderstorms with temperatures again in the low-80s.
Thursday – high pressure will finally begin to build in to our north by Thursday behind the cold front above. This will lead to better conditions, with partly sunny skies and temperatures in the low-80s.
Friday – partly sunny skies with high temperatures in the low-80s again with high pressure to our north.
Florence, Isaac, and Olivia All Pose a Threat to the US
These three storms have the potential to impact the Southeastern US, Puerto Rico, and Hawaii respectively within the next week. Among these, Florence is by far the most dangerous, as it could make landfall on North Carolina as a Category 4 storm with maximum sustained winds of 145 mph.
Hurricane Florence
Over the past day or so, the picture for Florence has become much clearer. Various forecast models have come into remarkable alignment on both the overall track and intensity of Florence over the next 5 days, which means bad news for the Carolinas. The picture looks grim, with Florence set to make landfall as a very strong Category 4 storm packing sustained winds of 145 mph. Florence has nothing but warm water ahead of it, and there should be minimal impact ot this storm from vertical wind shear. Even if the storm weakens due to eyewall replacement cycles prior to landfall, that would only serve to increase the radius of its damaging winds. No matter how you slice it, Florence is likely to bring deadly storm surge and risk of life-threatening flash flooding and inland flooding. The fact that its forecast track slows to a crawl after landfall makes the flooding risk inland especially serious.
Hurricane Isaac
Isaac is a compact storm, which is leading to some uncertainty regarding both its track and intensity forecast. Small tropical cyclones are subject to wild swings in intensity both ways, which can be unpredictable at times. As of now, Isaac is forecast to cross the Windward Islands of the Lesser Antilles as a Category 1 storm, then continue into the Caribbean where hostile shear conditions should cause it to weaken. It is worth noting, despite the UKMET model being an outlier, that there are some signals that the storm could turn to the northwest later in the period, which could mean a threat to Hispaniola and Puerto Rico.
Hurricane Olivia
This storm continues to track towards Hawaii. Although weakening due to the influence of moderate wind shear, it is still set to hit Hawaii later this week as a strong tropical storm. The primary threat to Hawaii from Olivia will be in the form of heavy rain and flash flooding regardless of whether it makes direct landfall on one of the main islands.
Cooler temperatures are finally upon us after a very hot and humid week. However, the cooler weather also brings a prolonged period of cloudy conditions, with chances for showers each day this weekend. Rain chances continue into the beginning of next week with a storm system approaching. Long-term, we look towards the tropics, as Tropical Storm Florence continues to have the potential for bringing some impacts to the area.
Rest of today – overcast, with much cooler high temperatures in the mid-70s. Chances for showers, especially later in the day.
Saturday – cloudy with high temperatures remain cool in the low-mid 70s with northeasterly onshore flow behind a cold front draped west-east south of us. This front (which passed through yesterday) will become stationary. Multiple impulses of energy will run along this boundary and allow for continued slight chances of showers.
Sunday – yet another cool, cloudy day with chances for rain. High temperatures around 70°F.
Monday – the stationary front stalled out south of us will return north as a warm front. As this happens, chances for rain increase, along with temperatures rising back into the mid-upper 70s.
Will Tropical Storm Florence Impact the East Coast?
Tropical Storm Florence has been active over the eastern and central North Atlantic for over a week now. Within the last few days, Florence underwent a rapid intensification that brought it up to Category 3 status, making it the first major hurricane of the 2018 Atlantic hurricane season. Subsequently, it has weakened to a tropical storm under relentless strong shear. However, it is expected to reintensify into a major hurricane once shear subsides. Worrying trends in the track forecast for Florence continue to allow for the possibility of landfall on the East Coast, or at the very least, substantial coastal impacts like dangerous surf and beach erosion. However, the National Hurricane Center continues to note that there is considerable uncertainty to the track forecast for Florence, such that it is still too early to determine what effects it will have on the East Coast. This storm will warrant careful observation over the coming days and I will have subsequent updates on it.
