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Why was Central Pennsylvania the bulls-eye for last week's heavy rain?

Updated: Sunday, October 20 2013, 06:28 PM CDT
Reported by Chief Meteorologist Tom Russell and Meteorologist Nate Wardle:

This week, we sat down to try to determine why the Harrisburg area was the focal point for the heavy rain last Thursday and Friday. 

The models were prepared for a large amount of rain, with one forecasting around 3-4 inches of rain.  However, that rain was closer to the coast with a sharp east to west gradient.  As we found out, the model underestimated the rain, likely due to many of the features discussed below, and also had the location of the heaviest rain too far east. 

We spent a lot of time researching this and looking at some different measures.  For a great, in-depth article complete with pictures, we recommend you check out this post from the Capital Weather Gang for The Washington Post: http://www.washingtonpost.com/blogs/capital-weather-gang/wp/2013/10/14/the-mid-atlantics-deluge-ghost-of-karen-pushes-rain-totals-well-beyond-predictions/

Overall, there were five factors that helped contribute to why we saw so much rain.

1)    The time of year:  The storm was an early-season Nor’easter, riding up the coast.  But due to the time of the year, ocean temperatures are still very warm, allowing for a lot of water vapor to be evaporated from the ocean and put into the clouds. 

2)    Easterly flow:  We knew that the area was going to be under easterly flow, but the strength of that easterly flow set up on the northern side of the Nor’easter.  That is why even when the storm was too our south, the focus of the moisture was over top us.  Because of the strength of this flow, as soon as the moisture supply rain out, the moisture was quickly replaced.  This explains why the rain never ended, and why breaks on the radar quickly filled in.

3)    The storm was cut off from another weather features.  As usual for this time of year, the Jet Stream is still off to our north, and there were no other factors, such as a front of another low or high pressure system to move the storm.  The long duration allowed the water to pile up.

4)    A stationary front set up from north to south east of the Appalachian mountains, allowing for moisture to converge over the area.   Winds met at this boundary, which was pretty much right over Harrisburg, allowing the air to lift which led to the bands of heavy rain.

5)    Shortwaves, or little waves of low pressure, rotated around the strong storm and when they swept through, they enhanced the rainfall.

Below are rain totals for the local area as reported to the National Weather Service:

THE FOLLOWING ARE UNOFFICIAL OBSERVATIONS TAKEN DURING THE
PAST 48 HOURS FOR THE HEAVY RAINFALL THAT IMPACTED CENTRAL
PENNSYLVANIA.  APPRECIATION IS EXTENDED TO ALL WHO
CONTRIBUTED TO THESE REPORTS.

LOCATION  STORM TOTAL     TIME/DATE   SOURCE
                     RAINFALL                     OF
                     /INCHES/            MEASUREMENT

...ADAMS COUNTY...
   1 SSE ABBOTTSTOWN     8.89   700 AM 10/12  COCORAHS
   2 N ABBOTTSTOWN       8.81   700 AM 10/12  COCORAHS
   10 W GETTYSBURG       8.00   544 PM 10/11  TRAINED SPOTTER
   1 NNE FAIRFIELD       7.58   800 AM 10/12  COCORAHS
   1 SE YORK SPRINGS     7.33   700 AM 10/12  COCORAHS
   YORK SPRINGS          7.31   700 AM 10/12  COCORAHS
   2 NNW FAIRFIELD       7.19   600 AM 10/12  COCORAHS
   CASHTOWN 1S           6.97   900 AM 10/12  CO-OP OBSERVER
   3 NE SABILLASVILLE 6.90   753 AM 10/12  AWOS
   2 WSW BIGLERVILLE     6.77   800 AM 10/12  COCORAHS
   5 E FAYETTEVILLE      6.75   230 PM 10/11  TRAINED SPOTTER
   HANOVER               6.66   900 AM 10/12  CO-OP OBSERVER
   2 SE GETTYSBURG       6.58   700 AM 10/12  COCORAHS
   3 NNW ORRTANNA        6.55   630 AM 10/12  COCORAHS
   3 WSW BIGLERVILLE     6.10   700 AM 10/12  COCORAHS
   5 SW GETTYSBURG       6.06   700 AM 10/12  COCORAHS
   BIGLERVILLE           6.06   900 AM 10/12  CO-OP OBSERVER
   6 SSW GETTYSBURG      5.47  1030 AM 10/12  COCORAHS
   7 SSE GETTYSBURG      5.10   700 AM 10/12  COCORAHS

...CUMBERLAND COUNTY...
   CAMP HILL            10.50   930 AM 10/12  SOCIAL MEDIA
   NEW CUMBERLAND       10.25   930 AM 10/12  SOCIAL MEDIA
   1 NNW MECHANICSBURG   7.91   700 AM 10/12  COCORAHS
   CARLISLE WATER PLANT  7.10   900 AM 10/12  CO-OP OBSERVER

...DAUPHIN COUNTY...
   MIDDLETOWN           10.24   522 PM 10/11  SOCIAL MEDIA
   HARRISBURG INT ARPT   9.73   800 AM 10/12  ASOS
   7 E HARRISBURG        8.68   700 AM 10/12  COCORAHS
   3 NW MIDDLETOWN       8.60   800 AM 10/12  COCORAHS
   HERSHEY               8.09   900 AM 10/12  CO-OP OBSERVER
   HARRISBURG            7.83   900 AM 10/12  CO-OP OBSERVER
   2 E PAXTONIA          7.78   700 AM 10/12  COCORAHS
   DAUPHIN               7.17   900 AM 10/12  TRAINED SPOTTER
   GRANTVILLE 5.73  1000 AM 10/12  SOCIAL MEDIA
   ELIZABETHVILLE        5.20   915 AM 10/12  SOCIAL MEDIA
   3 W LYKENS            4.90   525 PM 10/11  SOCIAL MEDIA
   DEHART DAM            4.72   900 AM 10/12  CO-OP OBSERVER

...FRANKLIN COUNTY...
   CHAMBERSBURG          6.90   500 PM 10/12  PUBLIC
   UPPER STRASBURG       5.86   900 AM 10/12  CO-OP OBSERVER
   SOUTH MOUNTAIN        5.69   900 AM 10/12  CO-OP OBSERVER
   WAYNESBORO            5.10   400 PM 10/11  TRAINED SPOTTER
   GREENCASTLE           4.58  1000 AM 10/12  TRAINED SPOTTER
   2 WNW GREENCASTLE     4.34  1100 AM 10/12  SOCIAL MEDIA

...JUNIATA COUNTY...
   EAST WATERFORD        4.80   800 AM 10/12  TRAINED SPOTTER

...LANCASTER COUNTY...
   WEST LANCASTER        9.60   435 AM 10/12  TRAINED SPOTTER
   MANHEIM               9.50   945 AM 10/12  SOCIAL MEDIA
   ELIZABETHTOWN         9.17   811 AM 10/12  STORM TOTAL
   MARIETTA              9.00   640 PM 10/11  TRAINED SPOTTER
   1 SSE SALUNGA-LANDIS  8.37   800 AM 10/12  COCORAHS
   LANDISVILLE           8.37   900 AM 10/12  TRAINED SPOTTER
   6 WNW LANCASTER       7.59   700 AM 10/12  COCORAHS
   MILLERSVILLE          6.49   900 AM 10/12  CO-OP OBSERVER
   TRUCE TOWER SITE      6.24   800 AM 10/12  IFLOWS
   COUNTY PARK           6.16   800 AM 10/12  IFLOWS
   SAFE HARBOR           6.12   900 AM 10/12  CO-OP OBSERVER
   3 S LANCASTER         6.44  1115 AM 10/12  TRAINED SPOTTER
   LANCASTER AIRPORT     5.70   753 AM 10/12  ASOS
   TRUCE                 4.83   800 AM 10/12  IFLOWS
   HOLTWOOD              4.25   900 AM 10/12  CO-OP OBSERVER

...LEBANON COUNTY...
   3 SSE PALMYRA         8.32   700 AM 10/12  COCORAHS
   ANNVILLE              7.05   920 AM 10/12  SOCIAL MEDIA
   3 WSW ANNVILLE        6.94   700 AM 10/12  COCORAHS
   LEBANON               6.50  1000 PM 10/11  TRAINED SPOTTER
   4 SSW NEWMANSTOWN     5.81   700 AM 10/12  COCORAHS

...NORTHUMBERLAND COUNTY...
   HERNDON               5.30   950 AM 10/12  SOCIAL MEDIA

...YORK COUNTY...
   GOLDSBORO            12.70  1100 AM 10/12  SOCIAL MEDIA
   CAPITAL CITY ARPT10.35   756 AM 10/12  ASOS
   YORK                  9.15   215 PM 10/11  TRAINED SPOTTER
   WSW NEW SALEM         9.13   500 AM 10/12  COCORAHS
   1 ENE VALLEY GREEN    9.12   600 AM 10/12  COCORAHS
   YORK AIRPORT          9.01   753 AM 10/12  ASOS
   3 NE YORK             9.00  1000 AM 10/12  SOCIAL MEDIA
   DALLASTOWN            8.85   920 AM 10/12  SOCIAL MEDIA
   4 WSW DOVER           8.66   720 AM 10/12  COCORAHS
   3 NNW YORK            8.39   700 AM 10/12  COCORAHS
   MOUNT WOLF            8.30   230 PM 10/11  SOCIAL MEDIA
   1 SE MOUNT WOLF       8.13   700 AM 10/12  COCORAHS
   WINDSOR               8.00   935 PM 10/11  TRAINED SPOTTER
   YORK HAVEN            7.52   900 AM 10/12  CO-OP OBSERVER
   SEVEN VALLEYS         7.50   315 PM 10/11  TRAINED SPOTTER
   SIDDONSBURG           7.35   524 PM 10/11  SOCIAL MEDIA
   SPRING GROVE          7.32  1200 AM 10/12  TRAINED SPOTTER
   DELTA                 6.80   735 PM 10/11  SOCIAL MEDIA
Why was Central Pennsylvania the bulls-eye for last week's heavy rain?


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