September 8-11 Final Discussion/Forecast

September 8-11 Coastal Rain Storm Final Discussion/Forecast

Quite a bit has changed in guidance and forecast in the last 24 hours. The storm is now just off the North Carolina coast around Cape Hatteras, which is on track with the previous forecast. The 12z/18z NAM give the mid atlantic a lot of rain as it moves the storm slow, it dissapates it further south hence not much rain gets into New England. The 12z/18z GFS have a further east rain swath with the delmarva and eastern Mid Atlantic getting the most rain as this model is faster with the storm and makes the second low heading right near or just on the coast.
12z HPC model discussion.

LOW MOVING UP THE MID ATLANTIC COAST/SECOND CIRCULATION IN THE OH VALLEY/APPALACHIANS... PREFERENCE: A BLEND OF THE 12Z GFS/09Z SREFMEAN/12Z UKMET THE INITIALIZED LOW FURTHER EAST LEADS TO THE LOW STAYING EAST OF THE 00Z FORECASTS OF THE NAM/GFS/UKMET/ECMWF THROUGH THE FORECAST CYCLE...WITH ONE SFC-700 MB LOW OFF THE COAST AND ANOTHER 700 MB LOW INLAND OVER THE APPALACHIANS/OH VALLEY. THE 15Z ANALYSIS SHOWS THE LOW POSITION ABT 50 NM NORTH OF THE NAM FORECAST LOW...SO ADJUSTING THE NAM FCSTS CORRESPONDINGLY NORTH APPEARS TO BE IN ORDER. THE 12Z NAM/UKMET ARE SLOWER MOVING THE LOW NORTH OVER THE COASTAL WATERS ON DAY ONE/EARLY DAY TWO WITH BETTER CLUSTERING AMONG THE 06Z-12Z GFS/00Z-12Z UKMET/00Z-12Z ECMWF ON DAY TWO...WITH THE 09Z SREF MEAN CLOSE TO THE NAM SOLUTION. ON DAY THREE...THE ECMWF HAD THE MOST WESTWARD PROGRESSION ACROSS PA WITH THE SFC LOW...AND THE CANADIAN FURTHEST NORTH. I RECOMMENDED GIVING LESS WEIGHT TO THE CANADIAN GLOBAL SINCE THE STRONG HIGH OVER NEW ENGLAND SHOULD BLOCK NORTHWARD MOVEMENT. WITH THE HIGH MOVING OFF THE NEW ENGLAND COAST...THE RAPID INLAND MOVEMENT OF THE ECMWF APPEARS TO BE OVERDONE. THE MODELS WERE IN GOOD AGREEMENT ON DAY ONE WITH THE EXISTING CIRCULATION IN THE OH VALLEY AT 700 MB DRIFTING SOUTH INTO KY ON DAY ONE AND SHEARING OUT. BIGGER DIFFERENCES DEVELOP ON DAY TWO-THREE AS THE MODELS DIFFER AS TO THE EVOLUTION OF THE CLOSED LOW OVER THE MID ATLANTIC AND WHETHER IT CROSSES THE APPALACHIANS INTO THE UPPER OH VALLEY/ADJACENT LOWER LAKES BY FRI. THE 12Z NAM REMAINS ON THE SOUTH EDGE OF THE SUITE OF 700 MB LOWS ON FRI...WITH THE ECMWF/UKMET/CANADIAN/06-12Z GFS/06Z-12Z GEFS MEAN/09Z SREFMEAN SHOWING THE CIRCULATION FURTHER NORTHEAST.

As of right now, I would go with a blend of the 18z NAM/GFS and 12z GGEM/ECM for this storm.

HPC is going with a more wet solution which I won't argue with, here is there short range discussion.

SHORT RANGE FORECAST DISCUSSION NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD 450 PM EDT TUE SEP 08 2009 VALID 00Z WED SEP 09 2009 - 12Z THU SEP 10 2009 AN AREA OF LOW PRESSURE NEAR THE OUTER BANKS OF NORTH CAROLINA IS EXPECTED TO MOVE VERY LITTLE DURING THE NEXT 48 HOURS. THE SYSTEM WILL REMAIN JUST OFF THE MID ATLANTIC COAST LINE AND GRADUALLY BECOME ABSORBED BY A COLD FRONT AND ASSOCIATED HIGH PRESSURE DESCENDING OUT OF NEW ENGLAND AND THE CANADIAN MARITIMES. EASTERLY LOW LEVEL FLOW BETWEEN THE LOW AND THE HIGH TO THE NORTH WILL CONTINUE TO PRODUCE CLOUDY...COOL AND EXTENSIVE RAINFALL ALONG THE OUTER BANKS...THE DELMARVA...SOUTHERN CHESAPEAKE AND DELAWARE BAYS. INLAND ALONG THE I-95 CORRIDOR FROM RICHMOND TO BOSTON...PERIODS OF LIGHT RAIN AND OCCASIONAL SHOWERS WILL BE THE PREVAILING WEATHER CONDITIONS DURING THE 24 TO 48 HOURS.

HPC day 1-3 QPF forecast is at the top of the post.

As you can see, the HPC is going is going with a widespread 1-3 inches in the Mid Atlantic with the heaviest rain in the delmarva area to southern New Jersey.
For my forecast and thoughts, once again I am going with the latest NAM/GFS solutions and the 12z run of the GGEM/ECM.

I am forecasting a slightly faster movement of the low[s] and the second further north hybrid low, subtropical in nature will form like the GFS is currently showing and should hug the coastline or go slightly inland while the original low stays further south and goes OTS. The faster movement we have, the more ground this storm will affect, so I am seeing a good widespread rainfall across the Mid Atlantic and Southern New England.

The major cities, Washington, Philadelphia, New York, and Boston look to get a general 1-3 nches of rain while to the northwest, there will be ligher rain less than 2 inches and a Trace to an inch in the interior regions, while the coastal Mid Atlantic area could see a good 2-4 inches being in the heaviest area of rain and close to the Hybrid Low.

Here is what I am predicting for the cities.
Richmond, VA:Around 0.5
Washington, DC:Around an inch
Baltimore, MD:1-2 inches
Philadelphia, PA:1-3 inches
New York, NY:1-3 inches
Boston, MA:Around an inch

With this, there could also be some breezy conditions around the coastal areas, in the range of 15-30mph locally, with that some possible coastal flooding and beach erosion.

That is my discussion and forecast on this storm, this is my final one irrigardless of what happens, everything you see above won't be changed, though some differences from guidance and forecasts could be possible.

Comments would be appreciated.