On Friday, we dodged a bullet in the weather world; a big coastal system brought snow, ice, and heavy rain to the Carolinas and southern Virginia, but the storm took more of an easterly course as it moved toward the DC Metro area. The Beltway stayed completely dry as a result, with places like Virginia Beach and Ocean City feeling the brunt of the rain and wind on Friday afternoon. Now we're keeping our eyes on another potential storm, slated for Wednesday and Thursday of next week. A lot could change between now and then, but let's take a look at the long-range model output for some clues!
If you've read my blog before, you know that I generally only look at the GFS (Global Forecast System, maintained by NOAA) and the ECMWF (European Centre for Mid-Range Weather Forecasts, maintained in the UK) for long-range forecasting. In general, I do not give favor or bias to one model over the other, though I will keep it in mind if one model out-performed the other in a recent event. For instance, the winter storm on March 3rd was not handled well by either model, but the European model did better as we got closer to the event. However, today's coastal storm and its eastward turn was correctly predicted by the GFS a few days in advance, whereas the Euro had the storm hammering us with almost an inch of rain, sleet and ice until Thursday night's model run, when it finally came on-board with the eastward turn.
Taking all this into account, I am currently leaning toward the GFS for next week's storm. The key for this one is in the jet stream. The jet contains areas that are favorable for storm development, and one of the key areas is called the left exit quadrant of the jet streak. The Euro has a very pronounced jet streak and classic left exit quadrant just to our south on Thursday morning, March 13th. Take a look:
The purple and red areas have the strongest wind, so the purple is the jet streak, where the winds are strongest. If you divide that streak into equal quadrants, you'll find the exit region is very close to the DC Metro. This puts us in a favorable location for cyclogenesis, or in other words, intense storm development. Hence, the European precipitation output for the same time period is pretty intense.
This model run shows the entire Metro getting between .25"-.5" of rain in just a 6-hour period on Thursday morning. The GFS model, however, is much more conservative with its rainfall for the Metro. Take a look at the wind forecast for the jet stream level on Thursday morning:
The red indicates the area with the strongest winds. You can see that this area is very small in comparison to the jet streak on the European model, and it doesn't contain that same signature of a left exit quadrant over the Metro. Instead, the winds appear to be stronger just to the east of DC, which actually makes our area much less favorable for cyclogenesis. As a result, the precipitation output is heavier to our north on the map.
The GFS has three times LESS precipitation for the DC Metro during the same 6-hour period on Thursday morning.
What these model forecasts do have in common, though, is the formation of a large storm. Both models also give us the potential for rain briefly changing to snow on Thursday, possibly leaving a light accumulation as colder air backs into the DMV. The storm's placement is still very much up in the air, and it's something we'll continue to keep an eye on for the next several days.