Parade of storms for early May
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Parade of storms for early May
A seemingly never-ending parade of weak storm systems will continue into the next week.

The weather pattern will remain active through the next week, as weak storms move quickly from west to east across the country. As these storms move across the country, they will be paired up with moisture from the Gulf of Mexico to produce rainfall every 24 to 36 hours over Central PA. This pattern is expected to continue into the middle of the month of May.

See the two images below.

The first is a representation of precipitation expected by a suite of atmospheric models run by NOAA at the National Center for Environmental Prediction (NCEP) known as the Global Ensemble Forecast System (GEFS). The plot shows both a running total of possible precipitation in Harrisburg through the next week (May 5-12) in green, and also the possible precipitation in every 6 hour period (grey lines at bottom). The left-hand scale is in inches, and time on the bottom is in UTC (or GMT, Greenwich Mean Time). The plot displays a constant train of rain storms every 24 to 36 hours over the region. None of these shots of rain looks extremely heavy, but the repeated rainfall could eventually create more flooding than has already occurred.

The second plot shows one reason why the forecast is so wet. It is an upper-level chart (again from the GEFS) that displays the expected heights of the 500hPa (hectoPascal or millibar) level - usually about 20kft up. The wind flow at that level parallels the constant-height lines displayed on the top chart (in bold black and brighter colors). Each of the colored lines is a solution from a different member of this ensemble of forecasts. The grey-shading on that same top panel portrays the relative (un)certainty of the occurence of this pattern. The light gray shades (less than 40 on the left-hand scale) mean that this outcome is fairly certain, and we can count on this being a solid forecast - even out at the 5th day.
The bottom panel in the second image shows a general lack of coloring (lots of white space, and little or no yellow, blue, etc.). This means that this is not an unusual pattern for this time of year.

This pattern boils down to a fast, generally west to east flow aloft. This flow should steer storm systems generally west to east across the country. A slightly southerly low-level wind flow will bring moisture up to a nearly stationary boundary or front near Pennsylvania (not shown).

All this spells a very unsettled and wet time for the Commonwealth right into the middle of May.
GEFS Ensemble Plume Diagram of Precipitation for 1 week (5 through 12 May 09)
GEFS Ensemble Plume Diagram of Precipitation for 1 week (5 through 12 May 09)

GEFS Spaghetti/Spread plot (top) and Mean/Anomaly plot (bottom) for Sun 10May2009 7PM EDT.
GEFS Spaghetti/Spread plot (top) and Mean/Anomaly plot (bottom) for Sun 10May2009 7PM EDT.

 
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