Monthly Archives: August 2021

Weather Updates

SUNDAY FORECAST UPDATE

Sun 6:30 PM Forecast Review — The forecast for Henri was a moving target for the past few days. Looking back, here’s what’s prominent: the GFS probably was the most consistent with the track. The Canadian and German ICON’s westward track was plain wrong and for a time, adversely influenced the official forecasts.  The areas of maximum rain Saturday night were close but still incorrectly placed. Last night’s NBM forecast for continuous rain here on Sunday was wrong.  Of the shorter range forecast models, the new HIRESW-FV3 probably did the best    

TROPICAL SYSTEMS NOW HAVE THEIR OWN WEB PAGE— Henri.
Hurricane Henri Forecast Updated Sat 11:15 PM here
The previously posted Weekend Weather Forecast is here

Forecast Update Sun 12:06 PM — The latest GFS just available shows the same lull in the precipitation for much of the rest of the daytime today as rain rotates around the upper low. The moisture source is an outflow band from Henri.

Water Vapor with superimposed GFS jet level wind streamlines showing moisture moving in north of us today. (Click on image for a larger view.)
Radar (MRMS- Multi-Radar Multi-Sensor from NOAA) showing the curvature of the precipitation will keep the precipitation chances low in Philadelphia for the balance of the daytime hours; the rain is rotating around our area. The western edge of this moisture plume will move back in about 3AM as the entire upper air low moves northeastward. The main precip shield of Henri is visible. (Click on image for a larger view.)

Rain moves in again about 3 AM Monday, similar to the NBM meteogram in the previous update.

Total additional rain is now about 0.8 inches, according to the GFS.

Forecast Update Sun 10:55 AM— With on and off rainfall now predicted through Monday morning, a NBM meteogram is the best way to convey the rainfree periods and most likely time for additional rainfall—

NBM 12z meteogram showing likely times of rain and thunderstorms (Blue Bell, PA) Anything greater than 17-18% (red line) is a good chance when working with 1 hour probabilities. (Click on image for a larger view.)

Forecast Update Sun 8:20 AM— Here is the current radar-based rain accumulation so far with this storm —

Radar-based rain accumulation totals as of 8:15 AM Sunday. There were areas of 3-6 “ of rain that has already fallen.

Note that the banding of the heaviest rain was north and east of where the models had predicted where the heaviest rain would fall. That’s probably as a good as they can do.

The latest NBM still shows considerable rain for today into early Monday, as the tropical moisture from Henri is expected to move in from the northeast. Areas north of Doylestown and near Allentown are predicted to have significantly more than 1 inch of rain additional.

NBM additional rain through early Monday morning.

Forecast Update Sat 10:38 PM — Tonight’s models just becoming available show a band of heavy rain to affect the Philadelphia area tonight. Current radar shows this moisture plume already working it’s way in.

With the exception of the NBM, most of tonight’s models have the heavy rain falling after midnight tonight and tapering off during Sunday morning.

Here are several of tonight’s models. Most have very heavy rain in the 3″ range except for the new HIRESW-FV3 which is based more on the GFS. (The actual GFS won’t be available until 11:44 PM, past my time on duty.)

It should be noted that the latest ICON model from this afternoon had backed down to 1.1″. While the amounts below are impressive, I’m not totally convinced that they’re not over-predicting rain.

NBM Rain Accumulation forecast through Sunday evening. (Click on image for a larger view.)
HRRR Rain Accumulation forecast through Sunday evening. (Click on image for a larger view.)
RAP Rain Accumulation forecast through Sunday evening. (Click on image for a larger view.)
NAM Rain Accumulation forecast through Sunday evening. (Click on image for a larger view.)
HIRESW-ARW MEM2 Rain Accumulation forecast through Sunday evening. (Click on image for a larger view.)
HIRESW-FV3 Rain Accumulation forecast through Sunday evening. (Click on image for a larger view.)

While it is clear that Hurricane Henri will not directly affect our area, there remains much uncertainty about the moisture flow when the tropical system enters Long Island and becomes an “extratropical” system.

There is currently a HUGE range of predicted rainfall in the Philadelphia area for Sunday.

The GFS model has close to 1 inch of rain falling Sunday in our area. The latest model blend (NBM) has 3-5 inches of rain, while the HRRR and RAP are in the 2-3 inch range. The NAM-NEST has less than 1 inch. The ICON has 1.1 inches and the Canadian GEM has 1.1 inches.

With the recognition that today’s showers were over-forecast, I’m going with the 1 inch and below range of rain through Sunday. But things could change.

The new models become available about 9:35 PM through midnight. I’ll update sometime after 10:30 PM.

Philadelphia Area Weather Forecasts

HENRI AND WEEKEND WEATHER FORECAST

TROPICAL SYSTEMS NOW HAVE THEIR OWN WEB PAGE— Henri.

Hurricane Henri Forecast Updated Sat 5:11 PM here

Saturday Forecast Updated Sat 10:39 AM highlighted below
Sunday Forecast Updated Sat 6:10 PM highlighted below

The coming weekend’s weather will be influenced by an upper air low pressure system, the residual tropical moisture in place and the moisture and eventual track of tropical storm (hurricane) Henri.

The Weekend Forecast

The weather this weekend is dependent upon the track of Henri. Even without Henri in the picture, the tropical moisture in place with the upper level low would give us a good chance of clouds, showers and thunderstorms. The weather with upper level closed lows are notorious for being difficult to forecast.

Saturday

A mix of clouds and sunshine in the morning. Showers and thunderstorms develop as early as 2-3 PM. The showers on Saturday are looking very scattered. Large model differences in location. Localized rainfall may be heavy. High temp 84.9º ± 2.1º NBM model, Blue Bell.

Sunday

Cloudy with showers/rain. Gusty winds likely. The amount of rain and wind will depend upon the track of Henri. Potential for heavy rain at the shore. High temp 83.6º ± 2.3º The NBM cranks out several inches of rain for our area. The GFS less than 1″ Will need to update later this evening.

High temp 79.4º ± 3.1º NBM model, Blue Bell.

I will be updating the forecast over the weekend. Check back for updates.

Philadelphia Weather Outlook

WEEKEND WEATHER OUTLOOK

Fri 12:38 PM — Some of this morning’s models are beginning to show a more westward loop in Henri’s track, possibly affecting our area more directly as it loops westward. An evolving forecast…

Fri 7:00 AM— Based on comparison of current water vapor imagery and model forecasts, it appears that the upper air low is further south than forecast by the models. This could have the effect of pulling Henri closer to the coastline. Just a conjecture. As for Friday as a ‘test forecast’, last night’s HRRR and NAM did the best with the radar location of this morning’s light showers.

The coming weekend weather will be influenced by an upper air low pressure system, the residual moisture of Fred, and the moisture outflow of tropical storm (possible hurricane) Henri. The result will be a complex forecast situation based on an unstable atmosphere with periods of sun, clouds, showers and thunderstorms.

GFS Cloud/radar/upper air forecast for Saturday at 2 PM. (CLOUDS tan-upper level clouds / maroon- mid level clouds / dark blue -low level clouds) Upper low over Virginia, Henri in the Atlantic.) Radar in usual blue green yellow orange. (Click on image for a larger view.)

Sky conditions will be similar to today (Thursday) with periods of sun, periods of dark clouds. The big unknown will be the exact timing and placement of showers/thunderstorms.

With daytime heating and weak non-specific triggers, the best bet will be that most of the shower activity will be in the afternoon and evening, when thermal instability is at its highest.

PWAT values will be in the high 1.9” to 2.1” range, meaning that any slow moving storms can drop significant rainfall in a short period of time.

Friday will be a good test of the models with this setup. Friday, the upper air low will trigger the development of a weak surface low just to our south. The models have showers moving in mid to late morning and lasting into the early afternoon. (From Philadelphia and south) We’ll see how things play out tomorrow.

Henri may strengthen from a tropical storm to a hurricane but is still expected to stay far enough off the NJ coast to only bring indirect effects of surf/wave effects at the shore and outflow moisture enhancing the already unstable atmosphere in place here. Of interest is that the ICON model has been showing the possibility of a jog westward and looping track as it gets to the eastern end of Long Island.