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.