Sun 04:27 PM Forecast Review — The clouds did move in about 3 PM, as forecast last night. One of these days, I’ll learn that the latest model isn’t always the best one and I’ll leave “well-enough alone”.
Update Sun 06/18 @ 9:42 AM — Last night’s models have sunny skies for today, so I’m going to drop the cloud forecast for the 3-6 PM time frame.
There is an upper air disturbance just northwest of Pennsylvania that doesn’t appear to be captured by last night’s models—
Sunday Morning Water Vapor Satellite image showing upper air disturbance northwest of Erie PA. It appears this will move north of Philadelphia this evening with just some clouds moving in about 6-7 PM. (Click on image for a larger view.)
Some cloudiness with this disturbance is expected this evening after 6 PM, mostly north of the city.
Sunday Forecast Update
Update Sat 6/17 10:36 PM — Tonight’s high resolution models show some similarity to today’s which correctly predicted the periods of cloudiness and the few pop up showers we had. (One missing ingredient tomorrow is a strong jet streak.)
So despite the model cloud algorithms again have little to no clouds for us on Sunday, I’m thinking that an increase in cloudiness with some isolated showers is a possibility, especially areas north of Philadelphia. Most likely time frame is 3-6 PM. Not a high confidence forecast with this change.
Update Sat 6/17 10:56 AM — Mobile quick update. This morning’s high resolution models show more clouds and pop up showers today than previously forecast.
Update Sat 06/17 @ 9:27 AM — There’s a bit more hazy/sunshine cloudiness than forecast by the models right now. A new download of the HRRR shows some potential hazy sunshine/cloudiness through the day. Not captured by the models’ built-in cloud algorithms—
Todays latest HrRR shows hazy sunshine though some potential cloudiness on Saturday. (Click on image for a larger view.)
Here’s the same with superimposed vertical motion—
12z HRRR relative humidity fields with vertical motion (red/yellow/violet areas) (Click on image for a larger view.)
Ordinarily, the built-in cloud algorithms for the HRRR and RAP tend to overstate cloudiness. Interestingly, when there’s strong cyclonic upper flow, they tend to understate it. The above shows relative humidity fields at different levels of the atmosphere above 50-60%. Combined with areas of vertical motion, we get some cloudiness and maybe some sprinkles.
Update Sat 6/17 8:42 AM — Looking at last night’s models, I’m returning to a modified original forecast for Saturday: Some instability cloudiness possible, but mostly sunny. An isolated sprinkle possible, mainly north of our area, remains a possibility. Becoming WINDY around noontime.
Update Fri 6/16 10:52 PM — Tonight’s models show no clouds or showers on Saturday. Sunny, but windy.
Some showers now in central PA may make it into the western suburbs later this evening, but are forecast to dissipate before reaching the city.
High pressure builds in for the weekend, but the upper air cyclonic flow will be with us on Saturday and significantly less on Sunday. (The re-established Omega block across the continent will keep us in some degree of upper air cyclonic flow until Wednesday or Thursday.)
Saturday
The upper air cyclonic flow keep things somewhat cooler than an average June day. Some instability cloudiness possible, but mostly sunny. A widely scattered sprinkle after 4 PM, mainly far north of our area, remains a possibility. Few areas will see them.
It it will be breezy to almost WINDY.
NBM wind meteogram (Click on image for a larger view.)
High temperature 79.6º sd 1.7º ( NBM model— location Blue Bell, PA)
Sunday
High pressure at the surface. Mostly sunny
High temperature 82.6º sd 0.7º ( NBM model— location Blue Bell, PA)
Fri 4:30 PM Forecast Review — Various areas did get rainfall today in line with the model forecasts from yesterday. Those models that emphasized the the rain mostly from the Delaware river east did an impressive job. (HRRR and NAM-NEST)
Here’s a MRMS based estimate of the rainfall received today—
MRMS total rainfall rain gauge/radar estimate through 2 PM. The line of heaviest rain was just a a mile or two west of the Delaware river, within the 2.5 Km resolution of the HRRR. (Click on image for a larger view.)
Update Fri 06/16 @ 1:28 PM — The rain moved in about 1 hour earlier than I had forecast.
Some additional showers possible later this afternoon.
An early look at the MRMS shows many areas in Philadelphia and eastward had 1-2″ of rain. The MRMS works on an hour so so delay, so I’ll post it later to provide a meaningful overview of the rain we received.
Live Blog Experiment
Today’s live blog was an experiment. It’s complicated to do and it won’t be a regular feature. But for some serious future weather events, I’ll consider activating it again.
Friday’s Storms
I’ve made several updates in a in short period of time today. You may need to refresh your browser to see the updates.
Update Fri 06/16 @ 9:28 AM —
Latest water vapor image shows the upper air disturbance in West Virginia that will rotate into our area by 1 PM. Dotted blue contours show areas of maximum moisture convergence—
(Click on image for a larger view.)
Update Fri 06/16 @ 9:24 AM — Latest HRRR (12z) just available shows 1PM maximum in instantaneous precipitation rate (PRATE) just west of Philadelphia —
12z HRRR PRATE at 1 PM (Click on image for a larger view.)
Update Fri 06/16 @ 8:28 AM — HRDPS forecast instantaneous precipitation rate at 1 PM—
06z HRDPS instantaneous precipitation rate at 1 PM. (Click on image for a larger view.)
Update Fri 06/16 @ 8:16 AM — Current Radar with likely track of heaviest rain—
MRMS radar composite with RAP model MaxOmega (vertical velocity pressure) and horizontal moisture convergence. (Click on image for a larger view.)
The latest HRDPS has joined last night’s HRRR in its forecast placement of the heaviest rain. It appears to be in South Jersey—
Today’s 06z HRDPS shows the heaviest rain in South Jersey. (Click on image for a larger view.)
Updated Fri 06/16 @ 8:02 AM —Fast Update: Not much change in the forecast. There’s still lack of clarity regarding the exact location of the heaviest storms.
For Philadelphia and the immediate western suburbs, the 12-2 PM time frame appears to be the period of maximum storm intensity. A second batch of storms possible 5 PM to 9 PM. There seems to be some emphasis on maximum rainfall just east of the Delaware River into Southern NJ.
I’m preparing some graphics…more info coming.
Friday’s Storms Update
Update Thu 6/15 11:03 PM — No significant change from the earlier forecast. The action starts as early as 10-11 AM in the western suburbs. Peaks about 1- 4 PM. Tonight’s models haven’t made it any easier to identify the areas that will receive the heaviest rain. Six models, six areas forecast for the heaviest rain. Trends— the storms may come through in two groups. PWAT values have increased. CAPE and shear values have also increased suggesting stronger storms.
Updates tomorrow morning.
Friday’s Storms Update
Updated Thu 06/15 @ 5:33 PM—
The high resolution models continue to forecast thunderstorms and rain for Friday, starting late morning (10-11 AM) in western suburbs and peaking between 1-4 PM in the city and adjacent areas of Southern NJ as a result of surface low pressure being spawned by an upper air low pressure system.
Due to low level moisture convergence, areas of very strong vertical motion and reasonably high PWAT (1.7″), the high resolution models crank out very heavy rainfall in some areas, on the order of 1″-3.5″.
(It should be noted that not all the models are predicting very heavy rain: the NBM and the GFS are showing as little as 0.4″ to 0.8″ of rain. )
As always, those models predicting the heavy precip differ in their forecast location of these heavier bands of rain. The HRRR has its maximum from the city eastward into South Jersey while the HRDPS and NAM-NEST have areas just to the west of the city receiving the heavier rain.
Much of this heavy rain will be the result of thunderstorm activity. While CAPE values are in the 1000 joules/kg range (moderate), moderately high levels of vertical shear (24-30/sec) and areas of low to moderate helicity (near 400), there is significant vertical motion dynamics forecast that will result in some strong, possibly severe storms.
NAM-NEST—
Today’s 18z NAM-NEST total rain forecast for Friday. (Click on image for larger view.)
HRDPS—
Today’s 12z HRDPS forecast for Friday accumulated rain. (Click on image for larger view.)
The latest HREF, just available, probably captures the best compromise, but may still be over forecasting the rain—
Today’s 18z HREF total accumulated rain forecast (Click on image for larger view.)
Rain Friday, Possibly Heavy
Updated Thu 06/15 @ 9:03 AM —
The persistent upper low over eastern Canada is forecast to spawn a surface low near our area on Friday. It looks like rain moves in during the late morning.
The models continue to show low pressure that may cause heavy rain in the immediate Philadelphia area, starting late morning (10-11 AM) and lasting through the afternoon. Instability will add thunderstorms to the mix. Severity levels to be posted later today.
As is always the case with precipitation, the exact location of the heavy banding of rain/thunderstorms isn’t often captured accurately by the models. Over the past 12 hours, there’s been wide range of locations forecast to receive the heaviest rain. (On the order of 2-3 inches!)
I’ve recently been impressed with the Canadian models for this current weather pattern.
Here’s the latest Canadian HRDPS model forecast for total rainfall on Friday—
Today’s 06z HRDPS showing a significant bullseye of heavy rain near Philadelphia. This is consistent with yesterday’s model runs. As illustrated last night in the graphic just below, the NOAA suite of models has more of an eastern axis for the heavy rain. (Click on image for a larger view.)
Updates later this afternoon.
Thursday – Friday Outlook
Update Wed 06/14 @ 4:31 PM— The system that moved through Wednesday was even slightly faster than the faster HRDPS had predicted. Sunshine broke out about 3-4 PM.
A beautiful day on tap for Thursday with temperatures 80º to 81º.
The persistent upper low over eastern Canada is forecast to spawn a surface low pressure system near our area on Friday. It looks like rain moves in during the morning.
This afternoon’s HREF and NAM-NEST are showing areas of heavy rain possible in and around Philadelphia. I’m not sure if that forecast will hold—
Latest NAM-NEST total rain forecast for Friday. This seems on the high side of things. (Click on image for larger view.)
For Saturday, spotty instability showers possible during the afternoon.
Sunday looks sunny.
It looks like the blocked pattern will remain through most of next week. I know I said last week we’d get a weather pattern change by the the middle of this week, but I didn’t think it would be just an eastward shifted block. We have gotten a change in the weather this week with several days of showers and rain.
It now looks like the blocked pattern will flatten by the end of next week, according to the GEFS. Don’t hold me to it.
Wednesday Radar
Rain on the way. Total rainfall amounts expected to be about 0.25″ in most areas, with some localized higher amounts.
Current radar at 8:42 AM showing RAP model superimposed data—
MRMS radar composite at 8: 42 AM with superimposed RAP model moisture convergence (blue lines) bullseye right over Philadelphia. Yellow lines are maximum vertical motion (Click on image for a larger view.)
Wednesday Forecast Update
Update Wed 06/14 @ 8:24 AM — The earlier noontime start to the showers now appears quite likely. In fact, some showers may move into western suburbs after 11 AM. There’s an increased likelihood of showers and some scattered embedded thunderstorms from 2 PM to 5 PM.
The latest HRDPS is faster with this system and has the much of the activity exiting by 4 to 5 PM with sunshine returning, while the NAM-NEST keeps some shower possibility until about 7-8 PM. I’m leaning towards the faster HRDPS.
06z HRDPS Precipitation Rate and clouds (black) forecast for 2PM (Click on image for a larger view.)
Showers & Thundershowers Wednesday
Update Tue 06/13 @ 7:44 PM — An upper low in NY State (part of the new Omega Block) will rotate an mid level (500 mb) disturbance through the Philadelphia area on Wednesday. The upper disturbance will be enhanced by southern jet streak energy—
Today’s 21z RAP model forecast for 5 PM Wednesday. Jet streaks (white arrows) and mid level vorticity disturbance (Red arrows) will rotate through late morning through the afternoon. (Click on image for a larger view.)
The combination of the jet streak energy, the mid level (500 mb ~18,000 ft) vorticity disturbance and thermal instability will set off scattered showers and thunderstorms.
Clouds move in late morning. Showers and some thundershowers possible as early as noontime, but most models have scattered showers and thunderstorms most likely at 4-5 PM and possible into the early evening. There will likely be some breaks of sun in the afternoon. Any sunshine will act as fuel for more thunderstorms later in the afternoon.
The forecast for this week has changed from just a few days ago. What had been expected to be a flattening of the long standing Omega Block has instead re-established itself with the block’s main axis shifted 30º more eastward than the previous block.
Additionally, a strengthening of the southern jet stream will introduce more moisture into our circulation than the previous several weeks. (The flattening of the block is now expected next week.)
GEFS shows a new, weaker Omega Block (white arrows) with increased presence of southern jet stream (red arrows). (Click on image for a larger view.)
What this means will be another week or so of cooler than average temperatures but with unsettled weather, and with a chance of more rain showers as low pressure at the surface develops where the northern and southern flows come together near the Carolinas—
GEFS forecast for Thursday evening. Persistent low pressure near Nova Scotia and the Carolinas and a persistent trough through our area. (Click on image for a larger view.)
Between the two low centers, a persistent trough will develop through our area, leading to the development of instability showers. We’ll see the first of these on Wednesday. Details later today
