Quite a different picture this weekend versus last weekend. Instead of summer like warmth, temperatures will be 10-20ºF below normal in the 50s-60s Saturday and Sunday. A series of slow moving coastal lows is the culprit, bringing much needed rain, and the most substantial rain since the beginning of the month. Memorial Day looks sunnier with temperatures climbing back towards normal in the 70s.
Rest of today – increasing clouds, high temperature in the mid-upper 60s. Rain begins to arrive towards the late afternoon and continues on right through the overnight hours. Overnight lows dropping to around 50ºF.
Weather Prediction Center surface forecast for 8 AM Saturday. A low pressure center moving from the Ohio Valley will elongate with a stationary front extending east-northeast offshore of the region. Waves of low pressure will the move along this boundary this weekend bringing multiple rounds of rain.
Saturday – windy, rainy, and raw, with temperatures only in the mid-50s, which is more typical of March than May. Persistent northeasterly onshore flow with a stationary front positioned south of the region and waves of low pressure moving along it will act to keep things cloudy and cool. Overnight lows again around the 50ºF mark with rain chances backing off but still possible.
Sunday – another low pressure moving along that stationary front will bring more chances for rain particularly early in the day. High temperatures slightly warmer in the low-60s. Overnight lows in the low-50s.
GFS model 1000-500 mb thickness and precipitation type for 11 AM Sunday. Another area of low pressure is depicted impacting the area.
Monday– the unsettled weather finally moves off with a return to sunnier weather. High temperatures Monday should hit the low-70s as a result. Overnight lows in the upper-50s.
A dramatic warm up in store this weekend with highs climbing into the mid-80s and the 90ºF mark. A couple of scattered chances for thunderstorms come with a weak shortwave Saturday and a cold front passing Sunday. This second front will cool things back to seasonable temperatures in the low-70s to start off next week. Yet another warm spell with 90s is taking shape for mid-week next week!
Rest of today – partly cloudy with high temperatures in the mid-upper 70s in the city as high pressure remains dominant, cooler at the coast with a sea breeze developing. Overnight lows in the low-60s.
GFS model 500 mb height and vorticity for Saturday at 8PM. A shortwave with attendant vorticity is visible across Southern New England stretching west-southwest into PA and OH.
Saturday – partly sunny with high temperatures in the mid-80s as a warm front pushes through. A chance for showers and thunderstorms mainly in the interior as a shortwave passes later in the day. Overnight lows quite warm, in the upper-60s, could break records for the warmest minimum temperatures for this day.
Sunday – mostly sunny with high temperatures warming to the 90ºF mark in the city, possibly warmer. Downsloping northwesterly flow should aid this warmup. An isolated shower or thunderstorm is possible later in the day with a weak cold front approaching. Overnight lows dropping to the low-60s.
GFS model 2-meter above ground temperature and 10-meter above ground temperature for 2 PM Sunday.
Monday– big difference in temperatures with high temperatures back in the low-70s behind the cold front passing on Sunday. Overnight lows in the mid-50s.
A wet, windy New Year’s Eve in NYC will give way to a very mild day to start 2019. However, for reasons outlined below, I don’t believe that record warm temperatures will be broken tomorrow. Strong winds will be a distinct possibility, though, and as such the National Weather Service has issued a wind advisory for the area. I think sustained winds could reach 30 mph, especially early in the day.
My Forecast
High: 57°F | Low: 36°F | Max sustained winds: 30 mph | Total precipitation: 0.04″ – verification for temperatures and precipitation will come from METAR data for the period between 1AM Tuesday and 1AM Tuesday (06Z Tuesday to 06Z Wednesday) at LGA. Wind speed verification will draw on the daily climate summary from the National Weather Service.
Verification
High: 60°F | Low: 40°F | Max sustained winds: 36 mph | Total precipitation: 0.02″ – this wasn’t a great forecast, aside from the precipitation. I was once again overly conservative on max wind speeds. Following layer mean wind analysis results would have yielded an almost perfect forecast here. For temperatures, I think the main issue was that the low level clouds I thought would materialize based on the forecast soundings available the day before just never came to be. This allowed high temperatures (and hence low temperatures) to become warmer than I forecast. In this case, perhaps it would have been prudent to consider that winds would be downsloping from the northwest, hence drying out somewhat at lower levels. This wind direction also favors clearing conditions following a cold front passage from the west.
Weather Prediction Center surface forecast for 7AM Tuesday
Synoptic Set Up
NYC will start the day off in the warm sector of a low pressure center tracking northeast through New England. This low will continue to deepen as it progresses east. The tightening pressure gradient will induce strong winds, and a trailing cold front will cross the region bringing about much colder air in its wake. Temperatures will peak early in the day and drop steadily through the day. The low temperature will occur late in the forecast period.
High Temperatures
Steady warm advection overnight with a warm front and warm sector associated with the parent low above will mean that temperatures increase to the mid-50s by day break. Forecast soundings from both GFS and MOS indicate, persistent, low overcast that I believe will be pivotal to keeping temperatures from going above 60°F. These low overcast clouds should block out enough sun to keep things a bit cooler than otherwise would be. Forecast soundings also show some upper level clouds that would help block additional sun. For these reasons, I’m siding with cooler NAM MOS guidance that calls for 55°F, and bumping up a couple degrees. Cold advection should take hold quickly in the afternoon in the wake of the trailing cold front. This will usher in westerly and northwesterly winds, which will downslope and warm a touch but will still overall bring colder air into the area. Once this cold advection takes place, temperatures should start to fall.
GFS forecast sounding for 9AM Tuesday, showing a well mixed layer up to about 825 mb and low clouds around 800 mb.
Low Temperatures
Because of the factors above, the low temperature should end up coming in late in the evening/overnight period going into Wednesday. I believe there’s not too much reason to deviate from MOS guidance which averages out to be about 35°F.
Max Sustained Winds
A well-mixed boundary layer is forecast to develop with pretty strong winds aloft in the late morning hours. A period of efficient downward momentum transfer should allow fast winds from above to mix down to the surface. Layer mean wind analysis suggests that winds could be as strong as 35 mph, however, I’m going a bit lower with sustained winds topping out at 30 mph since neither set of MOS guidance goes over 20 mph.
Total Precipitation
It’s clear at this point that the bulk of precipitation will fall before the beginning of the forecast period. However, both sets of MOS guidance, SREF and GEFS means all suggest some precipitation early in the overnight period. I’m going with a composite average of various sources at 0.04″.
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)
