Monthly Archives: November 2018

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

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

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