Update Fri @ 10:24 AM — A big feature of this system will be the windy conditions. Sustained winds near 20 mph with gusts in the 35+ mph range for much of Saturday.
Friday’s 12zNBM Wind Meteogram for Blue Bell, Pennsylvania (Click on image for a larger view.)
Update Fri @ 7:50 AM — The rain is moving in now from the west, on schedule. The latest GFS continues with the forecast for the heaviest rain to fall today, Friday through Friday night and into early Saturday. Then light rain to intermittent rain through early Sunday morning.
Total rainfall will be 2-2.5 inches with locally higher amounts. About 2 inches will fall during this earlier period.
Update Thu @ 10:46 PM — Tonight’s NBM, GEFS and NAM are showing the rain ending by Sunday late morning. Most show cloudiness lingering through the afternoon, especially areas south and east.
Update Thu @ 7:44 PM — 2-3 inches of rain expected Friday through early Sunday afternoon, with the greatest accumulations late Friday into early Saturday morning, according to the latest GFS.
Update Thu @ 5:53 PM — Rain is expected to move in from the west mid morning Friday (about 9 AM ± 2 hours). The ECMWF has rain starting as early as 6 AM and the Canadian GEM about noon.
Rain continues through Sunday morning. The models are coming together with clouds lingering but rain tapering and ending Sunday around noon or so. Some rain may re-develop Sunday evening.
I’ll have a better handle on this forecast tomorrow with my regular Weekend Weather Forecast.
Update Thu @ 9:36 AM — A quick update. The latest ECMWF just available continues the rain into Sunday early afternoon, especially south and east of Philadelphia. (The GFS has the rain ending early Sunday morning.) So there’s uncertainty whether Sunday will be a nicer day.
Latest ECMWF model just available shows rain lingering into much of Sunday, especially south and east. (Click on image for a larger view.)
Update Thu @ 9:04 AM — The expectation for clouds and rain for Friday and Saturday is still in the forecast. An upper level trough will slowly move eastward. Surface low pressure just to our south will bring an easterly flow of chilly air off of the ocean and rain. The rain looks to start as early as 7-9 AM Friday.
GFS Upper air wind (jet stream level 250 mb) forecast for Saturday at 2 PM. Deep trough over the eastern US with upper lows centers will spawn surface low pressure systems to our immediate south. (Click on image for a larger view.)
It still looks like we’ll have clearing on Sunday
Update Tue @ 8:30 PM — Not much change since yesterday’s posting. Very light precipitation amounts for Wednesday expected: 0.10-0.25 inches of rain, mostly coming in the morning hours .
The European and German models are predicting considerable rain for Friday through Saturday, on the order of 2″ of rain. The upper low looks to be just slow moving rather than quasi-stationary. As a result, Sunday should be dry with decreasing cloud cover.
It’s been announced that the NWS-NOAA will be upgrading their “Weather and Climate Operational Supercomputer System” referred to as “WCOSS” in a major way on or about June 27th.
This system does the actual data ingestion and weather model computations.
New Cray Supercomputers will almost triple processing power from 16 petaflops to 40 petaflops. This may result in weather models being available faster and in the case of evening models, earlier. It’s a giant upgrade and hopefully the transition goes smoothly.
(They are asking for forecasters to be alert for any systemic weather model errors.)
This week will feature a typical springtime weather pattern with plenty of showers and a quasi-stationary upper closed low in the forecast for the Philadelphia area.
The first low pressure system will move in for Wednesday—
GEFS ” statistical mode” forecast for Wednesday showing low pressure (1) directly over us with rain/showers. A second low pressure system (2) takes shape in northern Texas, expected to move in Friday through Saturday. (Click on image for a larger view.)
The second system moves in for Friday through Saturday, as a closed upper low becomes quasi-stationary just to our south.
Jet stream (250 mb winds) for late Friday/Saturday morning—
GEFS mode forecast 250 mb (jet stream level) winds showing closed low over Kentucky for Saturday 2AM. (Click on image for a larger view.)
An easterly flow make it chillier with this system. As is often the case with closed lows, the exact position of the upper low (likely to change by Saturday) will make all the difference. Right now, it looks like rain.
GEFS (statistical mode) forecast for Saturday at 2 PM. Upper low and surface low will combine for rain. Southerly track means chilly easterly wind off the ocean for us. (Click on image for a larger view.)
The inclement weather may linger into early Sunday with slow clearing.
The exact position of upper closed low forecasts are difficult to forecast and I expect things might change by the weekend.
High pressure will be with us for most of the weekend. An approaching warm front will bring some showers late Sunday afternoon or Sunday evening.
Saturday
High pressure will influence our weather on Saturday. It will still be somewhat windy mid-day. Highs near 65º.
NBM wind /wind gust meteogram for Blue Bell, PA- Friday through Sunday (Click on image for a larger view.)
Sunday
High pressure moves off as a warm front associated with a deep low in the Great Lakes region approaches. Today’s GFS and ICON models have showers moving in about 4 PM, but today’s ECMWF, SREF and Canadian RGEM hold off the showers until Sunday evening. So there’s uncertainty with the speed of this system. Highs 69 to 70º
Today’s 12z GFS shows clouds and showers moving in about 4 PM Sunday as a warm front approaches. (Click on image for a larger view.)
Weekend Weather & This Week’s Weather returns April 30th.
Thu 5:30 PM Forecast Review— We had the wind gusts and some areas had strong storms. The HRRR did pretty good forecasting the stronger storms north of Philadelphia.
NexRad radar loop about 5 PM. Courtesy of https://weathertap.com
Update Thu @ 10:15 AM — Some changes in the forecast with the latest HRRR just available. Timing of the frontal passage today is about 5 PM ± 1.5 hours in the immediate Philadelphia area with thunderstorms, then passing into NJ.
Today’s 12z HRRR simulated radar forecast for 5 PM. Shear vectors (long white arrows) are perpendicular to storm motion vectors (short white arrows), reducing chances of severe storms. Nonetheless, strong pressure changes and other air motion will result in high winds with the front itself. (Please note: model forecasts of simulated radar should not be taken literally. The model location of a thunderstorm is a model simulation for a moment in time, not reality.) (Click on image for a larger view.) HRRR wind gust forecast for 5 PM today. (Click on image for a larger view.)
A rapid change in pressure with the frontal passage will result in high wind gusts, as the winds shift from southwest to northwest. Depending on the model, gusts from 30-50 mph possible.
Previous model runs showed shear and helicity to be low with severe thunderstorms unlikely. Despite this, the HRRR is showing an elevated hail parameter with the storms with strong vertical motion being present.
Update Wed @ 6:08 PM — A cold front moves through Thursday afternoon with thunderstorms. Current timing is between 2:30 and 5 PM depending upon the model. Here’s the NAM-NEST forecast for 4 PM—
NAM-NEST forecast for 4 PM Thursday afternoon showing simulated radar and CAPE (shading). A line of thunderstorms is forecast to move through between 2:30 and 5 PM. CAPE values in the orange shading are above 1000 J/kg which is respectable, allowing for some storms to be strong.
Forecast helicity and vertical shear remains low and CAPE values remain in the respectable moderate range. No severe weather expected although some strong thunderstorms are possible.
Update Tue @ 8:08 PM — Very warm weather for Wednesday and Thursday with high temps 80-84º. (Average high is 64-65º.)
A cold front moves through Thursday afternoon, with current timing about 5 PM according to the latest NAM-NEST. Thunderstorms are expected.
A low helicity and low shear environment is currently predicted, so severe weather is unlikely, although some strong storms always possible since CAPE values are forecast to be in the respectable low moderate range of 600 joules/kg range.
Update Mon @ 6:50 PM — A review of the latest GFS and Canadian RGEM are more optimistic than the high resolution models that the showers tomorrow morning will be light and depart mid morning. Huge difference here, so I guess we’ll see.
Warm temps and sun in the afternoon.
Update Mon @ 5:11 PM — The low pressure system that gave us the showers and clouds over the weekend will rotate yet another weak disturbance and front through on Tuesday. (The center of this low is still near Nova Scotia.)
The forecast has changed from yesterday— what had been forecast to be a light sprinkle moving through early to mid morning Tuesday now looks to be a several hour period of showers, possibly lasting a bit past noon time. Total QPF is on the order of 0.15 inches.
Previously Posted Sun 7:32 PM —
Our weather will turn more spring-like this week as we come under the influence of an upper level ridge with warm air moving up from the south—
GEFS forecast for Wednesday morning. We’ll be under the influence of an upper ridge and a warm southwesterly flow. Temperatures will move into the 70’s and perhaps approach 80 Thursday, according to the NBM. The deep low in Minnesota will move into Canada and the cold front will dissipate as the low moves more northward. (Click on image for a larger view.)
We’ll have temperatures in the 70’s by mid-week and possibly higher. The cold front that will be causing severe weather in the middle of the country is expected to weaken considerably as it approaches us Friday.
No major storms expected this week for our area. Some showers Monday night will move out by Tuesday.
