Jun 29, 2013

The June 27th-28th Mohawk Valley Flash Flood Event

The very wet weather pattern continues across The Northeast and Mid-Atlantic States and it is not done with us just yet.

The anomalous pattern of moist tropical air flowing from the Gulf of Mexico and the Southwest North Atlantic Ocean up the East Coast to the Northeast is one reason why it has been so wet this month (and even last month, too).  Below is the GEFS (Global Ensemble Forecast System) forecast map for the forecast winds at 850 hPa and the +2 to +4 Standard Deviation (SD) of the wind at this level; the map is for late afternoon of the 27th.

The water vapor in the air was way  above normal for this time of year, with  SD values of +2 to +3 SD as shown in the map below.

When this moisture interacted with a strong low pressure system (for the time of year) then the rain was both widespread and very heavy especially when embedded convection occurred within the storm's rainshield.

The map to the riggt is the GEFS forecast map for the early morning of June 28th. The low pressure system was over Pennsylvania with a pressure anomaly of  -3 Standard Deviations (SD)  BELOW normal for late June.

The June 27th 8PM EDT sounding for Albany, NY is below. [ A sounding is a radiosonde that records temperature, moisture and wind speed and direction above the ground. Soundings are very useful in daily weather forecasting; the information from them can be used to:  forecast maximum temperature or precipitation type or amount; assess instability in the atmosphere (which is helpful in thunderstorm forecasting) to name but a few ways that this information can be used]. The analysis of this data [temperature trace is in red, dew point (moisture) data in dashed green and the wet bulb solid lighter green ] indicated that the airmass above Albany was convectively unstable and very moist from the ground up through a great height.  The sounding also indicated that the warm air extended above twenty thousand feet. This is significant; case studies of past flash flood events for the Eastern U.S., especially for areas east of the Appalachians, showed that heavy rain producing convection occurs with very warm air aloft. Cloud top temperatures of these convective cells are warmer than -62°C. This "warm topped" convection that formed combined with both larger scale and mesoscale features all interacted in time and place to cause maximum precipitation efficiency and the resulting heavy rainfall. [1]

While thunderstorms were forecast they would not be severe; however thunderstorms and embedded convective showers within the storm's rain area would likely produce downpours due to the warm and moist nature of the air. Various indices also indicated the instability of the air and supported heavier showers.

The Convective Hazards Assessment Program (CHAP) (aka Ricks Index or RI) is another analysis tool that uses from data soundings to assess both severe storm potential and for forecasting potential rainfall amounts.  The CHAP [2] analysis (not shown) indicated NO severe thunderstorm; the rainfall calculator (a part of the CHAP), indicated 4.76" of rain with a maximum potential rainfall of 5.58"  was possible.

The heaviest rains fell across the Western Mohawk Valley and Southern Herkimer County of NY State. Note the rain totals at Ilion, Little Falls, and Columbia Center and compare it to the CHAP  forecast precipitation.

The surface pressure anomaly, 850 hPA wind data and anomalies and the 500 hPa heights and anomalies (data not shown) indicated the threat for a flooding rain storm was quite likely. The use of ensemble and anomaly data is a very useful tool in forecasting "abnormal" or "extreme" weather events. If all the weather elements of moisture, wind and lift are forecast to come together in time and place and the anomaly data indicates these features have a significant deviation from normal  then the forecaster has a higher confidence in the outcome of the forecast and can even assign a probability of the event occurring..

This flash flood event "fit" a Gulf (of Mexico)/Tropical (moisture) Origins event type. [3]

Other Factors that  caused the Flash Flood [4]
  • Wet antecedent conditions across the Upper Mohawk River and the Western Mohawk Valley
  • The slope of the Western Mohawk Valley is very STEEP from the Adirondack Mountains to the north and the Mohawk Valley
  • Clay soil
  • Land use - Southern Herkimer County and Western Montgomery & Fulton Counties are farmland with some light industrial development

[1] Flash Flood Forecasting: An Ingredients-Based Methodology, Doswell, et al. Weather and Forecasting, December 1996/Vol. 11

[2]  Convective Hazards Assessment Process: Revised Ricks Index,  Ricks & Erickson AMS Storm Conference Preprints 2008

[3] The Use of Ensemble and Anomaly Data to Anticipate Extreme Flood Events in the Northeastern United States, Stuart and Grumm National Weather Digest

[4] From NWS Albany NY   Seminar for Broadcast Meteorologists May 2013

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.