Aug 10, 2016

Prolonged Wet Spell Still Likely

Much needed rain fall occurred across parts of the Northeast States on Wednesday morning helping to begin to ease the dry and drought conditions across parts of the region. Nonetheless a severe drought continues for most of the Northeast. Areas of west-central and western New York State as well as a good portion of Southern New England remain in moderate or severe drought as shown in the image below.


From the US Drought monitoring service here is the discussion regarding the Northeast's drought:

"Moderate to heavy rains, locally exceeding 5 inches, fell on areas from southwestern New England southward and eastward into the mid-Atlantic and through the central Appalachians. These rains removed dryness and drought from most of West Virginia and in the mid-Atlantic south of the Mason-Dixon line, but only scattered improvement was noted elsewhere. In eastern and northern New England, where only light rain fell, severe drought expanded through most of Massachusetts, and moderate drought covered the rest of southern New England. Rainfall deficits for the past 90 days range from 4 to 9 inches in much of this region. In addition, long-term rainfall shortages remain entrenched, with accumulated shortfalls over the past 2 years of 12 to 24 inches observed from central New England southward into northeastern Pennsylvania. Temperatures averaged a few degrees above normal this past week, exacerbating conditions."

However there is going to be a definite pattern change as it relates to our dearth of rainfall. As I mentioned in my update A Drippy Looking Long Range, the next 7 days upcoming will feature frequent bouts of  showery rains and thunderstorms with any shower or storm capable of producing torrential rain. Even though it has been dry and the rain will be absorbed by the ground, there is the chance that too much rain could fall in a short time period (like 2 hours or less) resulting in the threat for flash flooding. Definitely something to keep an eye on.  

Weather data is coming into agreement on both what periods during the next 7 days will be the rainiest and how much rainfall is possible.  For Eastern NY State and Western New England (our neck of the woods) the best chance for widespread rain  will be from Thursday into next Monday. The best chance for getting soaked will be during the afternoon hours right up through early night (10 PM or so). The exception to this will be this Saturday August 13th when the rain could start earlier and will probably continue well into the early morning hours of Sunday before it diminishes from NW to SE as Sunday moves on. This same weather data continues to indicate that precipitable water values over the next 5 days will be  near +3 standard deviations (SD) above  normal with values rising to near +4 SD on Saturday. These high deviations above normal strongly favor heavy rain. And once again several features will come together in time and place to efficiently make the heavy rain happen..

Here is the 7 DAY TOTAL AMOUNT of rain expected. (Forecast is from the Weather Prediction Center) It is for the 168 hour period from 8 AM Wed 10 Aug through next Wednesday, 8 AM 17 August:

The forecast map indicates a widespread 1.5-2 inch rain fall for us with more than a few locations getting 3 to even locally near 4". Keep in mind this the total expected rainfall for the next 7 days . BUT as for our region goes, I think that most of this heavy rain will occur in the Friday through Sunday morning period, with Saturday seeing the most rain.

Aug 7, 2016

A Drippy Looking Long Range

Long range forecast data as well as teleconnections and analogs are pointing to some potentially interesting weather for the Northeast states during the 10th-14th of August. We could be in for a prolonged period of wet and unsettled weather. Some of the unsettled weather could be in the form of possible strong thunderstorms. But like most severe weather threats much will depend on S.L.I.M factors, that is (wind) Shear, Lift, Instability and Moisture and how these ingredients come together in both time and place.

Severe weather possibility aside,  forecast data is indicating some high (octane) moisture swirling our way from late Wednesday into the upcoming weekend. Forecast data is indicating higher than normal precipitable water (PW) content over the Northeast US during this time period.

What is precipitable water (PW)?

PW is defined as the depth of water that would accumulate on the ground if all the moisture in the air above you fell as rain. So if PW in the column of air above has a value of let's say 1 or 2 inches then under prefect conditions you would see one or two inches of rain. PW values vary by day and season. By season there are "normal" PW values. So by comparing either the actual day's PW value or forecast value, one can assess the potential for significant precipitation.

Forecast PW departures from normal for the period Aug 10th-14th
The ensemble forecast  average is indicated on the charts above. This forecast value is then "compared" to normal PW values for the given dates. The comparison yields how many standard deviations (SD) ABOVE or BELOW normal the forecast PW values are. The greater the number of deviations the more anomalous the parameter is. The above data is taken from the GEFS forecast data sets and indicates that PW values will be near 2-3 SD above normal for much of this time, with SD increasing to +3 to +4 SD above normal on Saturday. These above normal SD values of precipitable water would tend to favor the possibility of a lot of rain, locally heavy on Wednesday, and then again from later Thursday through the first half of next Sunday the 14th.

To "wring" or condense the moisture out of the air the next weekend we'll need a lifting mechanism. . it looks like a stronger than normal (for mid-August)  low pressure system (by -2 to -3 SD; image above) will be moving across the Eastern States and provide the lift needed.

So for now the end of this week and next weekend looks like a prolonged period of wet weather is heading our way. We do need the rain, but there is the chance that some of us may wind up with too much rain in too short a period of time.

As of now forecast data sources indicate  locally heavy rainfall is possible on Wednesday with a better threat for more substantial widespread rainfall next Friday through Sunday. For now conservatively a widespread 1.5-2.5 inches could fall during this period but as always there could be locally higher amounts.

How high is the question. Small scale/mesoscale features like  thunderstorms: where do they form and how do they move could be factors in enhancing local rain fall amounts. Will these storms repeatedly form or move over the same locations?  Other features like outflow boundaries (which are caused by convection) and their interactions with other large and small scale weather features COULD lead to some local enhanced heavy rain amounts, too. This in turn could perhaps cause some local flooding issues.  The "small" and mesoscale systems are near impossible to predict days ahead but based on past experience, I think that they will show up as we get closer to the end of this week .

I'll be watching, as should you by keeping up on the latest weather forecasts.