If you've seen my forecast over the past couple days, you're probably dismayed about Friday's weather. Breezy, rainy, chilly. Far from an ideal way to start the weekend! The 2 main long-range models I use, the GFS and the European, are not in perfect agreement about the storm's track. And as you'll see, a small deviation makes a big impact on our weather!
First, the European model at 500mb (which, if you're a weather geek, you know is the level of non-divergence or "steering level" of the atmosphere). This is where we look for strong areas of positive vorticity advection; when in association with a synoptic trough, these areas of PVA are a sign of cyclogenesis and/or uplift.
As you can see, there's a huge, very deep trough over the Gulf Coast, but most of the energy is far away from the DC area. If you're having trouble spotting us, look to the right of the northernmost vorticity line in the open trough. This pattern looks very conducive to cyclogenesis- but not over the Mid-Atlantic. The GFS, on the other hand, paints a different picture.
The vorticity is extremely strong, just to the southwest of DC on the GFS in the same timeframe as the Euro prediction. Also, the vort maxes are placed clearly on the lee side of the synoptic trough. On the GFS, this system is cranking!
Take a look at how this manifests in precipitation. First, the Euro again, at both 12Z and 18Z (8am and Noon on Friday).
18Z Euro for QPF
The heaviest rainfall is out to sea and over the DelMarVa peninsula. In DC, the total precip for the day is less than .25"-- far from a big event. Compared to the GFS, it doesn't look that different as far as spatial distribution of the precip:
12Z GFS for QPF
18Z GFS for QPF
The GFS solution has the storm a couple hundred miles further inland, which makes a huge impact on our storm total precipitation. This model gives us about 2 inches of rain on Friday in DC! This would certainly cause minor flooding in low-lying areas, streams and creeks.
I know the European model has been getting a lot of press lately for being much more accurate than the GFS, but I would never discount an alternative solution entirely! Perhaps a solution that is something in between the GFS and Euro is what will ultimately manifest. Either way, it looks like Friday will be a nasty weather day throughout our area!