Monthly Archives: October 2018

NYC Weather Update – Weekend Nor’easter – Oct 26, 2018

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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.

WxChallenge for Omaha, Nebraska (KOMA) | Oct 16-19

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Last Week’s Results

Tough week last week, with the major forecast bust I had on precipitation for Oct 9 for Omaha, plus generally underwhelming performances on temperatures the remainder of the week. Since I did pretty well the on the first city, and am better this week, my cumulative rank hasn’t gone down too much and I’m sitting at #263 nationwide (out of 1500) as of this writing.

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
10/09/2018 57 50 16 0.74 22.7
10/10/2018 50 40 23 0.01 7.9
10/11/2018 51 38 19 0.00 8.0
10/12/2018 47 39 10 0.12 5.3

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 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 seasons when a forecast site could be on the cold side of a front with rain.

Days 2-4: Much better forecasting overall, especially in regards to precipitation. However, got a lot of error points for being too cold across the board with temperatures. Buying into the colder temperatures in MOS made for an overall forecast that was too cold.

Forecasting for Week 2 at KOMA (Omaha, NE) Oct 16-19

Synoptic Set Up

A much quieter weather week is in store for Omaha. With high pressure building, most of the forecast week will see sunny or clear conditions. Light synoptic flow suggests the possibility for winds decoupling at night, giving rise to highly favorable conditions for strong radiational cooling. Temperatures will be trending up through the week as a upper-level trough moves east and heights continue to build.

Notes

NAM MOS guidance seems to be the preferred alternative with respect to overnight lows, given that forecast soundings show a clear signal for nocturnal inversions. Our class discussion notes that 1.4” of snow fell at KOMA over the weekend. Temperatures are in the mid-40s today, but cold overnight lows may mean that we need to consider patchy snow cover in forecasting overnight lows. Since overnight lows are expected to be quite cold to start the week, daytime highs should also be below normal for this time of year. The NWS official forecast does show a warming trend in overnight lows through the week. Delta Method was suggested as a possible approach, and makes sense with how similar the air masses will be day to day. The trickiest part of the forecast this week will be how cold overnight with conditions conducive to strong radiational cooling, and just how much things can warm up during the day.

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)
10/16/2018 60 33 13 0.00
10/17/2018 58 37 7 0.00
10/18/2018 64 43 15 0.00
10/19/2018 69 49 16 0.00

WxChallenge for Omaha, Nebraska (KOMA) | Oct 9-12

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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.

WxChallenge for Portland International Airport (KPDX) | Oct 2-5

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Last Week’s Results

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.

Forecast Day Max Temp (°F) Min Temp (°F) Max Sustained Wind (kts) Precipitation (inches)
10/02/2018 68 54 11 0.05
10/03/2018 67 45 7 0.00
10/04/2018 66 46 7 0.00
10/05/2018 57 49 15 0.28