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WxChallenge Caribou, ME (KCAR) Day 7 High Temperatures – Nov 4, 2018

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What follows is a discussion lab that I wrote for this past week’s WxChallenge forecast competition as part of Penn State University World Campus’ METEO 410 capstone class in weather forecasting. I’m sharing this to give folks a glimpse into the forecasting process we’ve been learning, and because this discussion lab garnered some plaudits from my instructor for providing a really good analysis with attention to detail.

Model Guidance

12Z NAM MOS and 18Z GFS MOS (model output statistics) today agreed on a high temperature of 46°F for Day 7 (06Z Thursday to 06Z Friday). 00Z Wednesday’s NBM run was forecasting 44°F. 18Z EKDMOS shows ~46°F in the 50th percentile, with 50°F in the 90th percentile, and 41°F in the 10th percentile.

MOS forecasts October 31, 2018
National Blend of Models (NBM) – a consensus MOS product
EKDMOS (Ensemble Kernel Density MOS), producing a probabilistic forecast of severeal different variables. This shows maximum temperatures. The green bar shows the 10th percentile, red bar the 50th percentile, and blue bar the 90th percentile

Synoptic Set Up

By Thursday, the cold front of the occluded low that will bring precipitation Wednesday is forecast to have pushed through KCAR. During this frontal passage, winds will veer from the SSE towards the west. Winds are not forecast to be particularly strong, however, westerly winds would downslope a bit, enhancing wind speeds as well as warming temperatures a touch.

Weather Prediction Center (WPC) surface forecast for 00Z Friday November 2 (8PM Thursday, November 1, 2018)

Typically, we’d expect temperatures to be cooler behind a cold front due to cold air advection (CAA). Checking dynamical model forecast 2-meter temperatures, there’s not really evidence of large temperature gradients around KCAR. Even though winds will be blowing from areas of cooler temperatures towards warmer temperatures, the lack of a large gradient and low wind speeds do not suggest strong CAA.

It’s worth noting that the cold front appears to have anafrontal characteristics (precipitation behind the front seen in the WPC surface forecast) – this has implications on cloud cover behind the front. Both sets of MOS guidance show overcast conditions throughout the day. Forecast soundings suggest the main effect of the cold front is a drying out of the layer between approximately 900 mb to 600 mb initially, though by 18Z Thursday this layer dry layer tops out 700 mb (NAM has a smaller dry layer, between 900 mb and 750 mb – not pictured). Outside of this dry layer, clouds appear likely both near the surface and also from the top of the dry layer to as high as 200 mb (300 mb in NAM). In fact, it appears that the column above the dry layer will be saturated, and precipitation will be falling at upper levels during periods of the day, which explains why the dry layer shrinks from the top down as moisture works its way down through the column.

GFS forecast sounding

Closing Thoughts

Because of the likelihood of persistent, seemingly thick overcast during peak heating, I’m hesitant to side with the MOS consensus of 46°F, which I think is too warm. Even if precipitation doesn’t reach the ground, evaporational cooling may still be a factor. I think NBM’s 44°F is reasonable given the current data. I wouldn’t go too low into the low-40s because of warming impacts of downsloping westerly winds and the lack of any strong CAA.

Results

Subsequent MOS runs actually trended up, as high as 49°F. However, because of the factors outlined above, I continued to hedge down from MOS guidance, and submitted a finalized forecast of 46°F on the day. The actual high ended up being 45°F. Because I hedged down, I was able to minimize my error points for the day and ended up climbing to the top of the class leaderboard.

WxChallenge Philadelphia, PA Climatology – Nov 4, 2018

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As part of the WxChallenge competition and Penn State University World Campus’ METEO 410 capstone course on weather forecasting, we are required to write up climatologies for cities that we will be forecasting for during the competition. I thought I would share the latest one I put together for Philadelphia, PA, which will be our forecast city for the next 2 weeks in the competition.

Climatology for Philadelphia, PA (KPHL)

City Name / Station ID: Philadelphia, PA (Philadelphia International Airport, KPHL)

Time Period: November 6-November 16

Topography and Geography

Local Time Zone: Eastern Standard Time (UTC -5)

Station Elevation: 10 feet above sea level.

Station Location: Philadelphia International Airport (KPHL) lies on the north bank of the Delaware River, 6.75 miles southwest of City Hall in downtown Philadelphia.

Important Topographical Features: Philadelphia is located in the southeasternmost corner of Pennsylvania, along the border with New Jersey to the east defined by the Delaware River. Philadelphia lies along the Fall Line, and there are rolling hills oriented southwest-northeast immediately west and north of the city. These hills have elevations of 200-500 feet. The Appalachian Mountains are further north and west, though many of these can be characterized more as narrow ridges. The elevations of these ridges range from 1000-1500 feet. East of the city are lowlands of the coastal plain in New Jersey. Although KPHL isn’t directly on the coastline, there are significant bodies of water within 55 miles of the site, including Chesapeake Bay to the southwest, Delaware Bay to the south, and the Atlantic Ocean to the southeast and east. Lastly, although not technically a topographical feature, the city of Philadelphia is a sizable urban agglomeration that can have effects on local microclimates via differential heating (urban heat island effect).

Winds

Wind Roses:

Frequency (percentage) of the single most common wind direction: West-northwest, occurring around 11.5% of the time.

Directions that are most and least common: Most common wind directions: southwest (~10.25%), west (~10%), northwest (~9.5%), west-southwest (~8.75%). Least common wind directions: southeast (2.5%), east-southeast (~2.75%), south-southeast (3%).

Direction(s) most likely to produce the fastest winds: west-northwest, and northwest have the highest likelihood of producing winds in excess of 21.5 knots. Due west is not far behind either.

Direction(s) least likely to produce the fastest winds: The least common wind directions (east-southeast, southeast, and south-southeast) also are least likely to produce winds exceeding 16.5 knots. Among these, southeast winds have the lowest frequency of producing winds in excess of 16.5 knots.

Impacts of wind direction on local weather: Winds from the westerly-northerly directions flowing towards KPHL would all experience some degree of downsloping (not particularly strong), as they flow over and down the higher terrain in these regions as discussed in the section on topography. Southwesterly-easterly winds all have the potential to transport moisture into the KPHL area, as they would flow over Chesapeake Bay (southwest), Delaware Bay (south), and the Atlantic Ocean (southeast-east). Southwest winds are quite common – the southerly-easterly winds are significantly less common, but still occur collectively about 17% of the time. The LCD mentions both the Appalachian Mountains and the Atlantic Ocean as moderating influences, as winds from the former warm via downsloping; and winds from the advect cooler marine air in the warm season, and milder air in the cold season.

While northeasterly are generally uncommon, east-northeast winds are somewhat more frequent, occurring about 6.5% of the time. Winds from these directions are noteworthy for a couple impacts. First, when KPHL lies north of a deepening coastal low, these winds can enhance moisture transport from the Atlantic Ocean while also possibly bringing milder air from the ocean when the sea surface temperatures exceed surface temperatures during winter. Second, when a high pressure center approaches KPHL from the west, these winds can bring result in cold air damming as they would eventually pool cooler air at the base of higher terrain west of KPHL before turning south. This scenario would bring about cooler temperatures than otherwise expected. Though less of a concern during the cold season, there could be scenarios in which a strong enough sea breeze could penetrate far enough inland during the warm season to suppress temperatures at KPHL. On the other hand, the urban heat island effect induced by the city of Philadelphia should have year-round impacts in terms of generating an inbound wind from outlying suburbs towards the city center (which KPHL is very close to), while also resulting in warmer temperatures than surrounding areas.

Maximum observed two-minute wind speed for the month (or months) in knots: 40 knots (converted from 46 mph)

Temperatures

Date Normal Maximum (ºF) Normal Minimum (ºF) Record Maximum (ºF) Record Minimum (ºF) Record Lowest Maximum (ºF) Record Highest Minimum (ºF)
Tuesday 11/06 60 42 79 26 36 66
Wed.

11/07

 59 42 75 20 38 56
Thursday 11/08 59 41 78 25 42 61
Friday 11/09 59 41 78 23 40 60
Tuesday 11/13 57 40 72 24 38 57
Wed. 11/14 57 40 76 19 35 56
Thursday 11/15 56 39 81 19 38 61
Friday 11/16 56 39 76 22 38 55
RANGE 56-60 39-42 72-81 19-26 35-42 55-66

 

Precipitation

Date Normal (inches of liquid) Record Maximum (inches of liquid)
Tuesday 11/06 0.09 1.41
Wednesday 11/07 0.10 3.99
Thursday 11/08 0.09 3.07
Friday 11/09 0.09 0.86
Tuesday 11/13 0.09 1.56
Wednesday 11/14 0.09 2.64
Thursday 11/15 0.10 1.95
Friday 11/16 0.09 1.46
RANGE 0.09-0.10 0.86-3.99

 

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

WxChallenge for Portland International Airport (KPDX) | Sept 25-28

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Temporary Changes to this Blog

Over the next few weeks, I’m going to take a break from my normal posts about weather in NYC and the region because I’m participating in the WxChallenge national weather forecasting competition. This is a course requirement for the final semester of the Undergraduate Certificate in Weather Forecasting program with Penn State University’s World Campus that I’m enrolled in. During the course of this competition, I will be providing forecasts of high temperatures, low temperatures, maximum sustained winds, and total precipitation for various cities in the challenge. The first of these cities will be Portland, Oregon – specifically Portland International Airport (KPDX). If events of particular interest, such as severe weather, tropical storms, or etc are forecast to occur in NYC, I will make special posts regarding such events.

Forecasting for KPDX September 25-28, 2018

Synoptic Set Up (The Big Picture)

High pressure was located at the surface west of KPDX. A rex blocking pattern existed over the Pacific (a upper level high directly north of an upper level low, see animation below, the upper level high is west of OR/WA and shaded with some light blues, the upper level low just south of it shaded with oranges/reds). Some northeasterly winds were forecast during the forecast days, especially later in the day/overnight, which given the topography surrounding KPDX would lead to some downsloping. Still, northwesterly was forecast to be the predominant wind direction, which aligns well with climatological norms for September. A thermal low was mentioned to be in place through the mid-week period that would migrate daily from west to east.

Forecast Rationale

The synoptic set up favors little change in day-to-day conditions. Downsloping and the presence of a thermal low over the Willamette Valley suggest warmer than climatological and MOS (Model Output Statistics) guidance max temps. Despite mostly clear to clear skies throughout the forecast period, radiational cooling at night should be limited due to winds expected to be easterly overnight. When there are clear skies with no/calm winds, this allows infrared radiation to efficiently escape into space from the Earth’s surface, resulting in cooler temperatures when compared with a cloudy and/or windy night. Of note, easterly winds produce the fastest winds for KPDX due to some funneling effects from the Columbia River Gorge (think of this like blowing through a straw). This was referenced in the local forecast AFD. However, the MOS guidance didn’t really show a clear signal of wind directions favorable to strong funneling, so it would be unwise to go too much above guidance here. Late week cool down noted as the rex block breaks down and the upper low that was associated with makes its way towards the west coast. Exact placement/timing could bring some precipitation in the picture by the weekend and maybe early into week 2 forecast period.

Forecast Submitted

Forecast Day Max Temp (°F) Min Temp (°F) Max Sustained Wind

(knots)

 Precipitation (inches)
09/25/2018 82 52 15 0.00
09/26/2018 82 52 11 0.00
09/27/2018 85 54 8 0.00
09/28/2018 83 55 6 0.00