Updated Fri 08/25 @ 10:31 AM — The front moved though this morning.
This morning’s radar and RAP model overlay shows the showers have moved off. The lingering mid-level trough (dashed orange line) will move east and we’ll likely see a mix of sun and clouds for the rest of the day.
Last night’s forecast still holds:
Following the front, It will become quite humid with dew points in the low 70s by noontime. Some sunshine may break through late morning and early afternoon, but scattered showers are possible any time during the mid to late afternoon.
The only thing to add: Many of this morning’s models have little in the way of additional showers today. Additional showers, if any, will be widely scattered and most likely late afternoon or evening.
Friday’s Forecast
Updated Thu 08/24 @ 7:33 PM — A warm front will move through during Friday’s early morning hours (2 AM- 8 AM) with showers and thunderstorms. CAPE values are low, so no severe weather expected. However PWAT values are in the high (2.1″ to 2.2) range so some locally heavy downpours are possible, especially 5 AM to 7 AM.
Today’s 18z HREF accumulated rainfall forecast through noon on Friday. (Click on image for a larger view.)
Following the front, It will become quite humid with dew points in the low 70s by noontime. Some sunshine may break through late morning and early afternoon, but scattered showers are possible any time during the mid to late afternoon.
A cold front moves through Friday night with scattered showers. The cold front looks to be slow moving and not distinct. Only scattered showers or thundershowers expected at this time, but that may change.
Thu 4:22 PM Forecast Review — This morning’s forecast went as predicted, but the heavier band of showers moving through this afternoon wasn’t predicted by last night’s models.
Current Radar at 4 PM shows bands of rain not predicted. (Click on image for a larger view.)
Over the next few days, we’ll likely see that this won’t be the only forecast that goes astray.
As mentioned earlier this week “These sort of situations with stalled fronts and moisture moving over a ridge are difficult to forecast accurately. Small movements of the boundary will cause large differences in the amount of rain any area receives.”
We have the additional complications of a tropical system in the western Atlantic and a potential tropical disturbance developing off of the western Florida coast. For reasons I don’t understand (and my research has turned up empty), tropical systems in the picture seem to play havoc with model forecast accuracy.
Thursday Forecast
Updated Wed 08/23 @ 7:53 PM — A disturbance in the upper flow still appears to move mostly to our south and west tonight, along with the significant rainfall—
Water Vapor image with radar and RAP jet stream level wind streams (Click on image for a larger view.)
It will be mostly cloudy on Thursday with light showers mostly in the morning hours. Total rainfall through Thursday will be very light, especially from Philadelphia eastward, according to the HREF—
HREF total rainfall through Thursday at 8 PM. (The HRDPS has even lower amounts of rainfall from Philadelphia eastward.) (Click on image for a larger view.)
(Even lower rainfall amounts tonight and Thursday morning, according to the HRDPS.)
Rainfall from an approaching warm front Thursday night into Friday morning will likely be more substantial.
Wednesday Forecast Update
Updated Wed 08/23 @ 10:45 AM — The period from late Wednesday throughThursday is looking much less rainy than the HREF had forecast. Indeed, the HRRR and other models have moved towards the HRDPS which has been forecasting little in the way of rain here. (Showers move to our south and any showers moving towards us dissipate.) That’s why I began this week’s weather blog with “Unfavorable Pattern for Rain”
This morning’s 12z HRRR has joined the HRDPS in forecasting most of the significant rain will move to our south and north. (Some very light showers are still possible later tonight near the city. ) More significant showers possible Thursday night as a warm front moves northward. (Click on image for a larger view.)
Wednesday Forecast
Updated Tue 08/22 @ 8:36 PM — The cold front moved through early today and Tuesday was delightful. The frontal boundary has hit metaphorical wall and has advanced southward as far as the heat dome will allow. The boundary is pseudo stationary and impulses traveling over the top of the dome (as shown in yesterday’s graphic) will begin to affect our weather Wednesday.
Some of the moisture, possibly from Hillary, will advance over us with considerable cloud cover on Wednesday—
NAM forecast for Wednesday 3 PM shows clouds (dark) and rain advancing towards us over the heat dome. The wavy orange line (thickness line) is the approximate boundary of the hot/humid air. (Click on image for a larger view.)
These sort of situations with stalled fronts and moisture moving over a ridge are difficult to forecast accurately. Small movements of the boundary will cause large differences in the amount of rain any area receives.
Currently, our HREF model forecast is for rain moving in around late evening Wednesday and continuing through at least Thursday morning and possibly on and off through much of Thursday. Interestingly, the latest HRDPS has the rain totally missing our area and moving to our south.
I expect the Thursday through Friday forecast to have higher an average uncertainty, low confidence.
A Cold Front Pushes Heat Dome to our South
Updated Mon 08/21 @ 7:45 PM —We didn’t get as warm or as humid today as had been forecast, probably due to the layer of smoke in the atmosphere. (High in Philadelphia was 90º. Dew Points were in the mid 60s. Blue Bell on reached 86º.)
The heat dome is impressive on satellite imagery—
Water Vapor image shows the remnants of Hillary rotating around the large upper high pressure systems (heat dome) in the center of the country. The system in the Gulf is somewhat visible and the upper air support for the tropical system in the western Atlantic is also visible. (Click on image for a larger view.)
A sort of back door cold front (pushing in from the northeast) will suppress the heat dome to our south on Tuesday. Clouds in the morning Tuesday should break for partly to mostly sunny skies and cooler temperatures. Winds will shift to the NNE.
Several new things on the horizon—
Moisture rotating around the heat dome may bring us some showers Thursday into Friday.
A tropical system in the Gulf of Mexico will move westward, away from us, around the heat dome.
A tropical system looks like it will form in the western Atlantic somewhat near the Southeastern US. The path of this system is unknown at this time; it will likely remain offshore, but it may indirectly affect the Jersey shore within the week as its path bends north and northeastward.
Some of that Haze Monday is Smoke
Updated Mon 08/21 @ 9:16 AM — Today, some of that hazy sky isn’t only high cirrus clouds. It’s smoke and particulates, traveling across country over the northern edge of the ‘heat dome’ from Washington State Oregon and Canada. The haze/smoke layer is expected to thicken here about 4 PM—
HRRR smoke forecast for 4 PM today. Contours are low level smoke. (Click on image for a larger view.)
Hot, hazy and humid today. The smoke could shave off a degree from the expected high temperature. 91º to 93º still expected.
Unfavorable Pattern for Rain
We’ve moved into an unfavorable pattern for any rain, at least through Thursday.
We’re still under a northwest cyclonic flow of drier air moving over the northern edge of the ‘heat dome’ that’s in the Central US—
GEFS forecast for Monday evening. The general flow is from the northwest around the heat dome (white arrows). While a cold front will move through late Monday night, we’re again in an unfavorable region of a jet streak for rain. Little if any rainfall is expected with this cold front. Note the orange “thickness line” which denotes the edge of tropical air around the heat dome. It just moves through our area on Monday, only to be pushed back southwest. (Click on image for a larger view.)
The remnants of Hillary may bring much needed rain to NW parts of Canada as it flows due north and then bends towards us. We may see some of that moisture towards the end of this week.
Monday will be hot here with highs 90-93º—
NBM high temperatures Monday (standard deviation is 1.4º) (Click on image for a larger view.)
