Tuesday, August 06, 2013

The Return Of Moisture, The Demise (For Now) Of Dry

One of my favorite parameters in looking at heavy rainfall threats with approaching disturbances is PWAT -- precipitable water.  PWAT's aren't distant relations to an ewok although they sorta sound the same -- PWAT is a great use to look at not just surface moisture but the availability of moisture through the whole column of air in the atmosphere.

Think of PWAT's in a simple context -- if you could squeeze all of the moisture out of a column of air, it's how much rain would fall. The higher the number, the more moisture there is. Not just at the surface...but also aloft.

For instance, our region's airmass on Sunday and Monday was unusually dry for early August standards. The "average" PWAT around this time of the year is in the 1.4-1.5 range...we were running last night at 0.7, which is more typical for late September and early October. Our taste of early Fall was refreshing, beautiful, and a kind payback for the ark building many of us were employed to do in June and at times in July.

Unfortunately, such dry times do not last.  You can see the comparison between where we were (immediately below) on Monday night and where we're heading over the next few days.

Atmospheric moisture is on the increase over the next few days, with PWAT's steadily marching towards the 2.0 range in the region by the time we get to Wednesday evening (with tomorrow's threat for localized soaking storms) and, by the time we get to Friday, pushing 2.3 if one is to believe the GFS computer model.

Typically, when PWAT's approach or exceed 2.0 in the atmosphere during the summer forecasters can deduce the risk for heavy rain is pretty high if there's a disturbance or trigger that can make it rain. 2.0 is not a hard and fast guideline -- strong enough disturbances can yield heavy rainfall with lower numbers.  The eight inch rainfall in Philadelphia that occurred recently occurred with PWAT's in the 1.7 range.

Knowing that, the upcoming several days do provide the threat for heavy rainfall in the region. The GFS (above) and Euro show the prospect for a few inches of rain over the next five days...while the models themselves and each model run vary on the placement of the heaviest rains and how much may fall, the potential for some locations to receive three or more inches of rain in the next five days is pretty high.  Not everyone gets three...some will...and perhaps more than that...by Saturday.

That said, we've been relatively dry the past week and a couple of days, save for last Thursday, so a fair percentage of the region can handle some of the heavier rains expected. Three and six hour flood guidance for the Delaware Valley is relatively high -- with the exceptions the northwest suburbs and right along I-95 locally and across North Jersey.  Wednesday evening's round of heavier thunderstorms, should they materialize, will probably knock a dent into this to an extent...setting the stage for a higher risk for localized flooding with whatever moves through the region on Friday and Friday night.