Nov 24, 2013: Heavy Rain, Wind On Wednesday

Slightly revised 11/25 morning

Forecast Highlights:

f66The fall of 2013 ends with a much more active pattern, as a near record cold arctic air mass sits over the region today and tonight with highs in the 20s and lows in the 10s. The cold temperatures will moderate into Tuesday as a broad low pressure system organizes near the southern US, tracking up the eastern US and likely producing the first significant rain event to affect the region in over 2 months, along with a risk of strong winds. Cold temperatures will return for the late week, with temperatures generally remaining below average through the end of the month. (Image credit: PSU e-Wall)

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Saturday, November 23 Observations:

11.23.13A fast moving wave of low pressure moved through the region on Friday night, bringing a cold front through the area, with temperatures peaking around 12-2 AM on Saturday in the upper 40s to mid 50s across the area. Temperatures continued to fall through the morning hours before slightly warming up again, reaching a secondary peak in the afternoon in the low-mid 40s inland and the mid-upper 40s for the rest of the area. The day started out with partly sunny skies, but with increasing clouds as an arctic front moved through, bringing widespread snow squalls across the region, affecting parts of the area. These snow squalls were generally short lasting, but produced heavy snow with low visibility and strong wind gusts up to 40-50 mph. Along with the frontal passage, temperatures quickly fell into the upper 20s to low 30s, with the low temperatures occurring at midnight. Most airports in the area reported a trace of snow, with 0.2″ accumulating in LaGuardia airport and 0.1″ in Islip, NY.



Tonight – Monday: Near Record Low Temperatures

Frigid conditions were observed across the region today with an anomalously strong cold air mass, with partly sunny skies and daytime highs only peaking in the upper 20s to low 30s. Strong northwesterly winds were observed as well, around 15-30 mph sustained, with gusts up to 40-50 mph. Temperatures have started to gradually fall across the area, and will continue to fall with winds gradually decreasing through the evening and overnight hours. Low temperatures are expected to reach the low to mid 10s inland, mid to upper 10s in the north/west suburbs of NYC, southern CT and Long Island, and the low 20s in NYC. These temperatures will approach and/or potentially reach record lows for parts of the area, which are mostly in the upper 10s. Mostly sunny skies are expected for Monday with winds from the southwest and highs reaching the low to mid 30s across the area, which will be a few degrees warmer than today but still well below average.


Tuesday – Wednesday: Heavy Rain, Wind, Some Snow/Ice Expected

18z GFS at hour 66, for 12z Wednesday (7 AM), showing the first low affecting the region with heavy rain, and the second low over North Carolina (image credit: PSU e-Wall).

f66Storm Analysis: With the departure of the anomalous cold air mass, the next significant weather event will approach the region by the middle of the week. An upper level low currently near the southwestern US has been gradually tracking east, and will continue to move in that direction until reaching the Gulf of Mexico coast on Tuesday. Meanwhile, northern stream vorticity currently over western Canada will continue to dive southeast and amplify with ridging placed over western Canada, reaching the Great Lakes region by Tuesday. The northern stream trough will pick up the southern shortwave, with an expected phase that would lead to the most significant storm in over 2 months affecting the region.

Since yesterday’s update, the model guidance significantly narrowed the spread regarding the specific setup; the GFS, which until this point has largely been an outlier with its handling of the storm, has trended towards the rest of the model guidance and depicted phasing starting with its 12z run today, while the remainder of the models continue to show a significant storm affecting the region. Probability of precipitation has been increased to 100% across the area, as the uncertainty is now regarding the exact storm setup, track and impact. The overall consensus is for the storm to affect the region in two waves, with two somewhat separate low pressures becoming consolidated as the system moves out, with the first low tracking NNE either near or slightly west of NYC on Tuesday night into Wednesday morning, followed by the development of a second, stronger low near North Carolina on Wednesday morning which then tracks NNE through the region while quickly intensifying and becoming the dominant low pressure, consolidating as it quickly moves out of the region. Slight changes to the track are still possible, however, mainly with the second low pressure, which would especially impact the temperature and wind outlook, and the rain outlook to a somewhat lesser extent. Today’s update considers at least some separation between the low pressures, becoming consolidated as the low tracks near NYC, although it is possible the system ends up more consolidated with one elongated low pressure center, in which case impacts would be slightly different as explained below.

0z run of the high resolution 4k NAM at hour 48, for 0z Wednesday (Tuesday 7 PM), showing front end snow towards northern New York state and areas of freezing rain towards SE NY and NE Pennsylvania (image credit: PSU e-Wall).

cld48Precipitation Types: the onset of the storm is likely to fall as light snow in central-northern PA and western NY. Considering the storm will be in its developing stages as it moves through the region, however, strong southerly winds are expected in the 850mb-700mb layer with the mid level low placed to the west of the surface low, which will lead to a warm layer in the mid levels. With enough low level cold air in place initially, a changeover to sleet and freezing rain is expected by Tuesday evening over western PA and NY states as light freezing rain develops over central-NE Pennsylvania, interior NW NJ and SE NY. Given a lack of surface high pressure to the north, temperatures near the surface are likely to warm up as well, supporting a quick changeover to rain in NW NJ/SE NY by the early night hours, lasting a little longer over northeast Pennsylvania, with the ice/rain mixing line continuing to push northeast overnight as the first low pressure tracks NNE, resulting in most of the region changing over to rain except for far western PA and northern NY, where a wintry mix is likely to continue. As the second low pressure moves in on Wednesday, phasing will be underway with colder temperatures aloft entering the backside of the storm. There remains some uncertainty with the exact setup and track, although more widespread snow is favored in the western end of the low, currently likely to be placed near central PA-central NY but subject to change. Depending on the setup, a changeover to snow showers may be possible for NW NJ/interior SE NY as the storm moves out. The more widespread snow with the back end is expected regardless of the amount of separation between the two waves, but with the eastern extent of the snow dependent on the setup. Overall, the storm is likely to produce moderate snow and light ice accumulations primarily over the western half of PA/NY states and into northern New England.

Modeled precipitation totals on the 0z GFS, showing at least 2-3 inches of rain in the NYC area (Image credit: NCEP MAG).

gfs_namer_078_precip_p60Rain/Snow Totals: The next significant aspect of the storm is the precipitation totals, as the storm brings a significant amount of moisture up the East Coast. The NYC area has been under a moderate drought recently with a lack of significant rain events this fall – NYC has not experienced a rain event over 1/4 inch since September 22, with 0.25″ the 3rd biggest storm total this fall. While this storm will not completely eliminate the drought, it is expected to significantly lower the negative precipitation departures. With the first part of the storm on Tuesday evening into Wednesday morning, rain totals of at least 1 to 2 inches are expected. Totals with the second part of the storm during the day on Wednesday are subject to some changes, but at this time are likely to end up around 0.5 to 2 inches, adding up to a storm total of 1.5 to 3 inches, locally up to 4 inches. Regarding snow totals, a swath of at least 4-8 inches of snow is likely over western PA, western/northern NY, and northern New England, with locally higher totals up to 12″ possible, and a sharp cutoff in show totals to the east of this axis.

Wind/Temperatures: Windy conditions are expected as well with a strengthening low level jet. As winds near 925mb gradually intensify, windy conditions are likely to the east of the first low, mainly from NYC and further east, on Tuesday night into Wednesday morning with winds from the southeast at around 15-30 mph sustained, gusts up to or locally over 40 mph. With the anticipated track of the low pressure, a spike in temperatures is expected overnight as well in the warm sector, rising up to the mid 50s-low 60s from NYC and further north/east, while a sharp temperature gradient sets up along the storm track with temperatures likely staying in the mid-upper 40s towards western New Jersey and in the mid 30s towards Pennsylvania. The winds and temperatures for the rest of the storm depend on the setup of the second low, which is still somewhat uncertain at this time. The first scenario consists of some separation between the two storms and the second low tracking near or east of NYC, in which case the strong low level jet will remain offshore, with temperatures falling back into the 30s and 40s for the second part of the storm with increasing winds late on Wednesday due to the tightening pressure gradient. The second scenario consists of a more consolidated low tracking near or west of NYC, in which case the strong low level jet will be placed over and east of NYC, with 925mb winds of up to/over 80 knots likely; not all of those winds will mix down to the surface, although this setup would support wind gusts up to or over 50-60 mph. For tonight’s update, I am siding closer to the former scenario, with windy conditions east of NYC on Wednesday but not up to the maximum potential, although this is subject to some changes with the next update.


Forecast for NYC Area: The latest forecast is for light rain to develop on Tuesday afternoon and evening, starting off with light freezing rain towards interior NW NJ/SE NY with little to no ice accumulations expected at this time. The entire area will then change over to rain by the late evening hours as heavy rain falls throughout the overnight hours, with 1-2 inches of rain expected. Temperatures are likely to rise overnight into the mid-upper 40s inland, while spiking into the mid 50s-low 60s from NYC and further east, with wind gusts up to 40 mph possible east of NYC. A temporary lull in the heavy rain is possible in the morning with a continuation of rain otherwise likely through the late afternoon or evening hours with windy conditions possible again east of NYC, with falling temperatures and increasing westerly winds by the evening hours as the storm exits and rain ends.

Based on the latest forecast, storm rain totals of at least 1.5 to 3 inches are expected, with totals locally up to 4 inches possible. There remains some uncertainty with the exact setup of the storm, however, and some changes are still possible. The main areas of uncertainty are rain totals, which could be increased with the next update, and wind gusts, which could be increased especially from NYC and further east with wind gusts up to 50-60 mph possible in the stronger storm scenario. More information will be posted on the storm over the next few days.


Beyond The Storm: Cold, Dry Thanksgiving; Preview Of December Pattern

Behind the storm, another strong cold air mass will enter the region for the late week, with cold temperatures and dry conditions for Thanksgiving and into the weekend, generally peaking in the low-mid 30s inland and mid-upper 30s elsewhere for highs, with overnight lows falling into the 10s inland and 20s closer to and including NYC. The pressure gradient is expected to decrease on Thursday compared with Wednesday night, with NW winds likely around 15-30 mph, gusting up to 40 mph. Beyond the weekend, stronger ridging is likely to develop and persist near Alaska, supporting a continuation of the strong -EPO pattern into the first half of December. This is likely to drive additional cold surges into the US, although at this time with a -PNA/+NAO/+AO pattern expected to continue, the greatest probability of cold and stormy conditions is likely towards the central-western US, with temperatures in the northeast US region at this time likely to end up near average, probably starting out above average during the first few days or week of December. Occasional storm potentials are likely for the region, some of them possibly producing snow; even if the cold sets up more west than currently expected with a warmer than average start to the month, the overall pattern does not appear to resemble the previous 2 Decembers, which featured well above normal temperatures and little to no snowfall over a large part of the US.

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