Update Sun 12/18 @ 9:12 AM — Expect considerable instability cloudiness, similar to Saturday, to move in from the west about 11 AM – noon today. Also updated this week’s Winter Storm page.
NOAA and the NWS has just released the preview version (called the parallel run) of the new NBM model (version 4.1) this past Thursday, and major improvements in precipitation type forecasts suggests that I lean heavily on this new NBM model for the storm’s precipitation type forecast.
From National Weather Service /NOAA—
Improvements in NBM Model 4.1 – inclusion of additional Australian Global and Canadian models and NOAA’S new HIRESW-FV3–
“Improvements in winter weather guidance for snow, ice, freezing rain, and unconditional precipitation type through the increase in [ensemble] membership (18 to 100 members) and leveraging direct model precipitation type guidance averaging direct model precipitation type guidance.”
“Usage of high-quality Quantitative Precipitation Estimates (QPE) observational and long sampled climatological data sets (e.g., the Multi Radar/Multi Sensor System (MRMS) and Multi-Source Weighted-Ensemble Precipitation (MSWEP) for calibrating probabilistic quantitative precipitation forecasts”
Update Sat 12/17 @ 5:30 PM — We did have the cloudiness today. Sunday will likely be similar to Saturday regarding clouds, possibly even cloudier in the afternoon. Colder. Forecast below still holds.
Update Sat 12/17 @ 5:33 PM — Winter Storm page updated for storm expected Thursday to Saturday.
Update Sat 12/17 @ 10:31 AM — Last night’s models are less emphatic about cloudiness this afternoon.
Update Fri 12/16 @ 5:51 PM — Updated Saturday and Sunday forecast below to reflect new model forecasts. Changes are highlighted.
Our weather this weekend will be influenced by a slow moving upper air low and a slowly departing surface low that gave us the rainy weather Thursday into Friday. High pressure will be building in, but disturbances rotating around the upper low will cause windy conditions and some occasional cloudiness.
Water Vapor image shows upper air configuration—
Water Vapor Image Friday morning shows the upper low approaching the Great Lakes area. The arrows depict the general location of the jet stream. (Click on image for a larger view.)
Friday’s 12z GFS forecast for Saturday at 5 PM. The upper low hasn’t moved that much eastward and the surface low is still off the coast of Maine. Cold high pressure will be sinking southward into Missouri. The circulation around the upper low will continue to affect us. (Click on image for a larger view.)
A colder pattern is setting up for the weekend and the coming week with cold high pressure pushing the jet stream to the south of us.
I’m keeping my eye on significant coastal storm that is forecast to form in the late Thursday through early Saturday time frame. The impulse expected to trigger this storm is way off in the Pacific right now and it’s too soon to consider it a reality.
At the same time, this is the first time in a long while where a white Christmas here is a distinct possibility. It currently looks to be a rain–> snow storm for us. Stay tuned.
Previously Posted Fri 12:07 PM —
Saturday
Mostly sunny in the morning, considerable instability cloudiness in the afternoon, especially west of the city. Windy/breezy and cold. Wind chills in the low 30s.
High temperature 40.1º sd 1.8º NBM model – for location Blue Bell, PA
Sunday
Sunny and cold. A period cloudiness in the early afternoon. Still breezy. Wind chills in the 20s!
High temperature 37.0º sd 1.8º NBM model – for location Blue Bell, PA
One reader wanted to know how to get the latest hourly probability of precipitation for our area.
One of the best ways is a direct link to the NBM (Model Blend) text output. The NBM is considered the best model for rain probability and the model data from this link is updated every few hours. (Yes, the NBM model is updated every hour, but to get the absolute latest, you’d have to dive into the bowels of the NOAA NOMADS server.)
I’ve pre-configured the URL for you to provide the model output for Philadelphia Airport, Wings Field Blue Bell, and Northeast Philadelphia airport—
The one you want to focus on is the NBH ( H= hourly).
It provides hourly probability (P01) in percentage (on an hourly basis, anything greater than 18% and increasing is meaningful) and the amount (Q01) in hundredths of an inch.
It’s important to know, that with accumulated rainfall, the number refers to amount having accumulated in the preceding hour.
The tricky part is converting to the UTC time to Eastern Standard or Eastern Daylight time.
The example below is for KLOM, Wings Field, Blue Bell.
So in the example below, this is the NBM model run is from 0700 UTC (3 AM EDT). The first row (labeled UTC) are the forecast hours and the column labeled “14” is the UTC TIME forecast time. So 14UTC = 10 AM EDT (or 9 AM EST).
Looking down the 1 hour probability (P01) is 48% and the quantity of rain fallen in the preceding hour (Q01) is 2. 2= 0.02″
Beware that for some of the other data displayed, especially in the NBE, the meaning can less less than intuitive. In the NBE, the column labeled, as an example, 00 day FRI, refers to the 12 hours PRIOR to 00z Friday which means daytime THURSDAY!
MIXED SLEET/FREEZING RAIN THURSDAY NIGHT CHANGING TO ALL RAIN BY MORNING
Fri 10:05 AM Forecast Review — So, we had even more ice accumulation than I would have guessed and temperatures did fall at or below freezing well into NJ, as forecast by last night’s NAM-NEST and HRRR. The URMA shows temperatures at 2 AM—
URMA from 2 AM Friday morning (URMA= “UnRestricted Mesoscale Analysis”) The URMA represents actual temperatures and conditions and is used for model verification. Since it represents corrected observations, the URMA is is released with a six hour delay to allow for corrected data input. (Click on image for a larger view.)
While the changeover to rain occurred, there was continued ice accretion past the rise in air temperatures above 32º. Most of the models over-predicted the rate of temperature rise. The major global models (GFS, ECMWF) never showed the temperatures drops into NJ.
Update Thu @ 9:55 PM — The precip is developing and moving in as forecast. Insignificant forecast changes with tonight’s models. Rain may linger until late morning.
Update Thu @ 5:54 PM — Here’s the latest trends based on today’s models. The trend is for some freezing precipitation to occur further southeastward than previously forecast (see image below). The changes are italicized:
The main area of precipitation, a mix of snow/sleet and freezing rain starts between 8 and 11 PM this evening. The heaviest frozen mixed precipitation will be between midnight and 2 AM.
Some areas north and west may have some accumulation/icy coating of frozen precipitation. The amount of precip falling in the frozen form may be as much as 0.3 inches water in northwestern areas.
The mix transitions to all rain in our area by 3-7 AM Friday morning.
Rain continues until mid morning, then ends.
Today’s 18z NAM-NEST precipitation type forecast 11 PM tonight. The 32º contour line is well into NJ. (Click on image for a larger view.)
Update Thu @ 9:27 AM — Here’s the latest trends which are consistent with last night’s forecast:
An area of light sleet or snow showers may move in during the afternoon, ahead of the main bolus of precipitation. (No accumulation expected.)
The main area of precipitation, a mix of snow/sleet and freezing rain starts between 8 and 11 PM this evening.
Some areas north and west may have some accumulation/icy coating of frozen precipitation. The amount of precip falling in the frozen form may be as much as 0.4 inches water in northwestern areas.
The mix transitions to all rain in our area by 4-7 AM Friday morning
Latest HRRR showing the greatest extent of the icy precipitation southward at 1 AM Friday. By 2 AM, the HRRR has the icy precip moving northward and all rain by 7 AM in our area.
This morning’s 12z HRRR showing the southeastern most extent of the its predicted frozen precip at 1 AM. (Click on image for a larger view.)
This morning’s NAM-NEST and HIRESW models are colder and have the extent of the mixed frozen precip falling as far southward as the Delaware River. I think the NAM-NEST may be too cold and I’m sticking with the HRRR forecast above. All models forecast a change to rain by 7AM.
Update Thu @ 8:02 AM — Last night’s models continue with a rapid warm up of the lower atmosphere between 3 and 5 AM. Any mixed icy precipitation will change to all rain by 7 AM in most of the immediate PHL area. The Canadian RGEM has joined the pack with a warmup.
Update Wed @ 9:25 PM — Tonight’s models have an area of mixed precipitation moving through in the early afternoon in advance of the main bolus later in the evening.
Update Wed @ 6:02 PM — Most of today’s models have continued with the following trend—
A mix of snow/sleet and freezing rain starts between 8 and 11 PM Thursday.
Some areas north and west may have some accumulation/icy coating of frozen precipitation. The amount of precip falling in the frozen form may be as much as 0.4 inches water.
The mix transitions to all rain in our area by 5-7 AM Friday morning.
The HRRR, NAM-NEST, HREF are on board with this warmup towards daybreak.
There’s one fly in the ointment: The Canadian RGEM (which has done the best with recent transitions from rain to snow behind cold fronts) is showing significant “cold air damming” and delayed transition to all rain. It shows freezing temps at 8 AM.
Here’s the latest RGEM forecast for 8 AM Friday—
Canadian RGEM forecast Precip Type 8 AM Friday. It has a much colder profile. (Click on image for a larger view.)
I’ll be interested to see if this trend holds and whether the RGEM had the right forecast.
By the way, I recently came across a table that shows which models the NBM weighs more heavily in determining precipitation type (rain/snow/sleet/freezing rain)—
Model weights in percentage used by the National Blend of Models (NBM) for its determination of conditional probability of rain/freezing rain/sleet/snow. Note that it mostly uses the HRRR and NAM-NEST followed by the other HIRESW models. The Canadian RGEM is referred to by its other official name, the “CMC RDPS”. It isn’t weighted very highly, but the RGEM has been revised/updated subsequent to the NBM weighting being decided. (Click on image for a larger view.)
Update Wed @ 10:08 AM —The forecast period is now in the range of the higher resolution models. The concern about icy conditions early Friday morning appears to have somewhat abated as the latest models show less cold air at night and a more rapid warm up by daybreak Friday.
While the models are still predicting several hours of sleet mixed with rain in some of the area late Thursday evening into Thursday night, the trend has been to forecast a rapid increase in temperatures above freezing by daybreak Friday in much of the area.
The latest HRRR model and shows sleet and freezing rain at 1 AM in the northern and western suburbs. These same areas are ALL rain by 6 AM, according to the HRRR.
This morning’s 12z HRRR at 1 AM Friday morning showing sleet and freezing rain north of the 32º degree contour. This changes to ALL rain by 6 AM according to the HRRR. (Click on image for a larger view.)
I’m leaning towards the warmer HRRR.
The Canadian RGEM model (the model recently upgraded and improved in December) has done very well this year with cold front transitions of rain to snow. It has a colder, icier forecast, but still with a transition to rain about 7 AM close to the city—
Canadian RGEM forecast for 7 AM Friday. (Click on image for a larger view.)
I’ll continue to follow this…
Update Tue @ 8:40 PM —Today’s models show the following trends—
The models had trended colder this morning, but the afternoon’s runs have trended slightly warmer.
Most models have a wintry precipitation mix starting as sleet/rain between 7 PM and 10 PM Thursday evening.
(The NAM has some light sleet in the afternoon before the main slug of precipitation moves in during the evening.)
Temperatures in the immediate Philadelphia area do not support much if any ice accumulation on roads or other surfaces.
Most models have the sleet and any freezing rain changing over to plain rain in the immediate PHL area between 5 and 8 AM Friday.
Areas of concern are north and west of the 32º freezing surface temperatures (white contour)—
Tonight’s 00z NBM model precip forecast for 1 AM Friday morning. Most areas have mixed frozen rain/sleet mix at 1 AM Some snow north of Quakertown. Areas north of the 32º freezing line may have hazardous conditions from ice accumulation by daybreak. (Click on image for a larger view.)
I suspect things might change. I’ll update tomorrow
Update Tue @ 8:51 AM — Last night’s models continue with the forecast of sleet, freezing rain and rain mix for much of our area, starting Thursday evening and changing to rain Friday early to mid-morning, with a changeover delayed in far northern suburbs.
This morning’s 06zSREF showing precipitation type at 4 AM Thursday morning. (Click on image for a larger view.)
Some significant ice buildup is looking possible in northern and western suburbs by daybreak Friday. Stay tuned.
Update Mon @ 7:52 PM — The latest ICON model (below) shows the complexity of the storm expected to develop Thursday into Friday morning. Heavy precipitation is currently predicted by all models, especially far northern suburbs. The latest NBM and GEFS are trending warmer, which would reduce the chance of heavy ice or sleet buildup.
Today’s 18z ICON model shows low pressure developing along the coast at 1 AM Friday morning. A very complex system with multiple low pressure centers. The ICON shows significant precip with the temp at or below freezing from Philadelphia north and westward. (Click on image for a larger view.)
Update Mon @ 5:37 PM — A possible significant ice/sleet storm is setting up for late Thursday into Friday.
A cold front will move through the area Wednesday dropping our high temperatures back to near and below seasonal levels. The front will stall not far to our south. Cold high pressure will set up to our north behind this front, while the southern branch of the jet stream brings very moist air into the mix.
GFS forecast for Thursday 4 PM. Cold front boundary stalled through Virginia while cold high pressure is to our north. Moisture moving up over this stalled front will cause precipitation to fall through a cold air layer near the surface. Sleet and freezing rain expected. (Click on image for a larger view.)
This arrangement is referred to as a classic “cold air damming” setup — cold air remains at the surface and is slow to retreat due to strong high pressure to the north, while warm moist air forced to ride above the cold surface layer at 3000 -5000 feet causing mixed ice and sleet type precipitation.
The models are notoriously poor at predicting the warm up at the surface in these setups; they often predict warming too early, under-predicting the ice and freezing rain.
The latest NAM is fast with the sleet (and briefly snow) moving in. It has some frozen precip as early as Thursday afternoon.
Today’s 18z NAM forecast for Thursday at 4 PM. Significant area of sleet and freezing rain shown in our area.
Today’s ECMWF has significant precipitation falling in our area Thursday night while temperatures at the surface are below 32º. Not a good situation.
The current model blend (NBM) is warmer with temps above freezing, posing less of a threat for an ice storm in the immediate PHL area.
I’ll keep an eye this. Stay tuned.
Previously Posted Sun 4:54 PM —
A warm up is expected Monday in advance of a moist southerly jet flow with rain on Tuesday as another deep low forms over the Great Lakes area in response to phasing northern and southern jet streams—
Today’s 12z ECMWF jet stream level winds (300 mb) showing strong flow from the south and phasing for a deep low over the Great Lakes on Tuesday. (Click on image for a larger view.)
Following rain on Tuesday, Wednesday will initially be mild temperature-wise, but somewhat colder air will filter in late Wednesday.
Another storm is expected late Thursday, although the timing is uncertain; some models have this next storm moving through Friday.
The Thursday/Friday storm will be rain for us, but the rain-snow line will not be that far to our north—
Today’s 12z ECMWF forecast for Friday 10 AM. (Click on image for a larger view.)
