Jun 5, 2013

More on our Thursday-Saturday Heavy Rainfall

Confidence continues to grow in the region getting a long duration of rain  event with heavy totals for many of us. The rain will arrive by Thursday June 5th and continue into Saturday the 7th before it winds down.

Trends in the NWPs

Over the past 24 hours Numerical Weather Prediction (NWP) models are converging on a solution of both the evolution of this complex low pressure system (complex meaning multiple surface low pressure systems  and disturbances aloft) and on the amount of moisture that will be overspreading the region during this time.

Again the overall duration of this event will span over 48 hours with the periods of heaviest rainfall occurring in two periods - Thursday afternoon and night and then around midday Friday into late Friday night with the greatest greatest amount of rain probably falling on Friday.

Thursday's rains

NWP data for Thursday evening June 6th shows that the rainfall will be going full bore (maps below). This first round of heavy rain will be associated with an area of low pressure moving from the Ohio Valley to NY State. The Canadian GEM-GBL (GGEM) is the strongest with this low. ALL the NWP suites for Thursday night have a stalled area of HIGH pressure centered near Newfoundland with its associated axis or ridge of high pressure extending south across the Atlantic Ocean, Between this high and the complex area of low pressure over the Continental US is a strong and extensive warm moist airflow from BOTH off the Atlantic and from the Northwest Caribbean Sea and Gulf of Mexico (GOM)

By Friday morning NWP data has this first low moving passed the region with the steadier rain tapering off to showers for a few hours. Forecast amounts of Precipitable Water (PW) [data not shown] continue to be  +1 to +2 SDs above normal with  a strong S and SE wind at the surface up through about 10,000 feet AGL.  (Please refer to my Previous blog post on what PW is and what the SDs are: 
Heavy-rain-expected-end-of-week ) 

Friday into Saturday AM rains

By Friday night our attention will be focusing on the second wave of heavy rain fall heading our way. Like Thursday's rains this one will be associated with some upper air disturbances and a surface low pressure system moving NNE along the East Coast. This low while initially of tropical origin WILL not be tropical in nature as it interacts with surrounding non-tropical features. However it WILL have quite a bit of moisture associated with it, thus the call for more locally heavy rains. Below are the NWP suites for Friday night.

For Friday night through Saturday morning forecast PW and anomalies (maps not shown) are at their greatest, on the order of +2 to +3 SD  (with +4 anomalies possible across all of Southern New England).
At the 850 hPa  level (5,000 feet AGL) the S-SE wind anomalies are on the order of -2 to -3 SDs, again indicators of a strong flow of moisture advecting across the Northeast States.

How Much Rain?

I'm still forecasting a widespread near 2 inch TOTAL rainfall for Thursday through Saturday morning for all. Still feel quite confident in higher amounts (approaching 4" perhaps more) especially across the S and SE facing slopes of the Catskills, Adirondacks, Taconics Berkshires and Greens.

Good news the rain should wind down and end by Saturday afternoon and Sunday will be the best day of the weekend, dry sunny and milder. Don't blink on this day because you might miss the nice weather it looks like more rain heading our way by Monday and Tuesday for sure.