Jun 27, 2013

Very Heavy Rain + Thunderstorms = Flood Threat

Once again we are dealing with more rain this Thursday Night into Friday morning and it could be VERY heavy. All the atmospheric elements appear to be coming together in both time and place for an intense period of rain falling across Eastern NY State and Western New England during Thursday Night (June 27th) through Friday Morning (June 28th). The heaviest rain is expected to fall during late Thursday Night through early Friday AM. The rain is associated with anomalously "deep"/strong area of low pressure over southwestern Pennsylvania.
Surface Map Northeast US
Here is a forecast map of the low for early Friday morning (6/28/2013) and also how "unusually" low the central pressure of the low is for late June. The pressures at the surface with this feature are -2 to -3 standard deviations BELOW normal.

And it is already causing an expansive area of rain, heavy across PA and Central and Eastern NY State.
Albany Radar 9:44 PM 6/27/2013
A flow of air from the south, from near the surface up through 500 hPa,  (with the air moving over us having "originated" from the Gulf of Mexico and Southwest North Atlantic Ocean), is laden with an anomalously high moisture content. 
To the left is a forecast chart of the precipitable water (PWAT) along with its "departure from normal". The PWAT chart shows water vapor in the atmosphere over for early Friday morning running 2-3 standard deviations above normal.

When combined with the "stronger than normal" southerly wind flow aloft (chart to right) with standard deviations of +3 to + 4 and the unusually "deep" low pressure system moving across the area tonight we have the ingredients for a heavy rain event coming together in both time and place.

The combination of "unusual" departures from normal with surface  features, upper air features and thermodynamic features (PWAT) both observed and forecast all fit the consensus of a summertime heavy rain event. 

The probability for at least an inch of rain across Eastern NY State over the next 18 hours or so is high (likely) 70-80% . [Map to right]

As for actual forecast rainfall amounts many factors will come into play. Not only all of the above anomalous features but also local affects like orography (terrain) with a SSE flow higher elevations across the east facing slopes of the Catskills, the Berkshires and Greens, as well as the southern and southeastern Adirondacks and across the Taconics will get enhanced rainfall amounts due to the flow of the moist air being nearly perpendicular to these mountain ranges. Also embedded convection - thunderstorms will increase rainfall amounts locally. There is the potential for hourly rainfall rates throughout the region on 1"-2" / hour over night tonight into Friday morning. The rain will taper off from south to north during Friday morning. Due to the expected heavy rainfall during early Friday morning the potential for FLASH FLOODING is HIGH! Then as the rain tapers off from south to north between 8AM and Noon Friday, the potential for river flooding across Central and Eastern NY into Western New England is high. River flooding happens due to runoff from smaller streams and the higher terrain working its way into the larger and/or main stem rivers. This river flooding will be most likely to occur during late Friday morning through late Friday night or Saturday.  

Map to the left is the expected total rainfall for the Northeast from HPC  by 8PM Friday evening June 28th. I think its not high enough across the higher terrain of Eastern New York and Western New England where I am forecasting 2-4 inches, even some point/spot amounts to 5" not out of the question. Across the Hudson Valley a solid 1-2 inches is expected  (with a few spot 3 inch amounts possible).  Because of tonight's and Friday's expected heavy rainfall a FLOOD WATCH has been posted for much of the Northeast, including Eastern NY State and Western New England.

Be safe everyone! Remember if you are driving and encounter a water covered road TURN AROUND! DON'T DROWN.