Morning e-mail from Jeff: After record heat on Saturday, a weak cool front has in fact moved off the coast ushering in an usually dry and “cool” air mass for the first week of July. Dry air continues to settle southward this morning with dewpoints falling into the 50’s and 60’s across the region which is rare for July. Mid level temperatures are also cooling and while the surface dry air will heat up quickly only looking at the mid 90’s today and likely a little cooler on Tuesday. With dry air in place, overnight lows will fall toward those dry dewpoints or into the 60’s across the area. While it will still be warm during the day, the lack of significant humidity will make it feel near the actual temperature or similar to a “dry heat”. Main part of this forecast will focus on a major upcoming pattern change that promises a significant increase in rain chances toward the end of the week and next weekend. Large upper level trough over the MS valley this morning will be forced westward (retro-grade) due to building Atlantic ridging over the western Atlantic. This trough axis will only slowly move westward taking nearly the entire week to reach eastern TX. While there is some significant difference in the model guidance on where this axis resides by the end of the week and where a possible mid or upper level low develops over the southern plains…the overall trend is toward a very wet pattern. At the same time as this trough axis drifts westward, a strong tropical wave will move west out of the Caribbean Sea and into the Gulf of Mexico with a pool of deep tropical moisture (PWS 2.2-2.4 inches) flooding much of the central and eastern Gulf by late this week. This moisture will be drawn northward on the eastern side of the trough axis bringing potentially significant rainfall. Current thinking is that the area will salvage a decent 4th of July as the trough axis will likely be just east (near the TX/LA state line) on Thursday with dry air still in place over our region. Tropical air mass will be lurking not far away and expect it to arrive into the coastal waters late Thursday and on the coast Friday with rain chances increasing south of I-10 on Friday. Brunt of tropical surge arrives Saturday into Sunday with PWS climbing to well over 2.0 inches and trough axis/mid level low developing/moving into central TX placing our region under very favorable upper air divergence and lift. Patterns such as these can produce some incredible rainfall in short periods of time and while rain chances will be increasing and peaking over the weekend and a flood threat may also develop. Mid and upper level low pressure systems can at times begin to act like tropical cyclones when embedded within a tropical air mass…this usually results in very heavy concentrated rainfall near the core of the system overnight and in the early morning hours and then banding of rainfall on the fringes during the afternoon hours. Rainfall rates in such a moist nearly saturated air mass will approach or exceed 2-3 inches per hour. Models have also been bouncing around with attempting to close off the trough wave/trough axis over the western Gulf and this would not be out of the question given the likely extensive amounts of thunderstorms that will be ongoing off the TX coast Friday-Sunday. Regardless if a surface low did form, the impacts would be nearly the same with copious rainfall across SE TX into the coastal bend.
Posted on: Mon, 01 Jul 2013 13:24:13 +0000
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