STORM2K

Change on the Horizon‏

After over a week of nice fall weather, changes are on the way.

High pressure aloft over the region and ridging SW from the eastern US has kept a fairly dry and warm air mass in place for the past several days. However high pressure at the surface is moving eastward and low pressure is developing in the lee of the Rockies ahead of a slow moving upper level trough over the SW US. This is resulting in an increase in the southerly flow and slight increase in surface moisture. At the same time powerful Hurricane Miriam is moving NW off the western coast of MX under the influence of ridging over N MX and the developing trough over the SW US. High level moisture plume is clearly noted extending NE from the circulation of Miriam spreading ENE to NE into central MX and toward the TX border.

Parameters are slowly coming together for a period of very wet weather Fri-Sun as the upper trough over the SW US interacts with eastern Pacific hurricane Miriam. The overall setup will likely produce an excessive rainfall event for some location across the state of TX late this week/this weekend. Current upper level trough will slow with lift reaching TX by late this week, while both Gulf and mid/high level moisture increase across the area as Miriam sheds moisture and energy NE. PWS increase toward 1.5 inches on Thursday and 2.0 inches by late Friday and then into the 2.0-2.4 inch range on Saturday with soundings becoming increasingly saturated. Upper level jet dynamics come into play by Friday as a 250mb jet streak is progged over NC TX increasing large scale lift over C and SE TX. Expect to see scattered showers and thunderstorms develop and move inland along the TX coast with more organized rainfall over C and N TX on Friday. With moisture levels running very high expect areas of very heavy rainfall.

Upper trough and moisture plume from Miriam sag eastward over the weekend with SE TX coming increasingly under the influence of both the lifting by the trough aloft and high level moisture/energy from the tropical system. Expect more widespread and organized rainfall potential Saturday into Sunday. With moisture levels in the “excessive” range heavy rainfall is likely, but at this time there is no well defined surface boundary to focus/anchor convection at least not at the present time. One would think that with upstream convection some sort of outflow boundary could/would be present on the following days and this potential will need to be watched.

Since we are still over 48 hours from the onset of any event, there is plenty of time to fine tune and nail down the location that looks most favorable for heavy rainfall. Only recently have model guidance come into decent agreement on the phasing of the moisture of Miriam and the upper level trough so some additional changes are likely.

Portastorm wrote:The QPF forecast out of the Hydrometeorological Prediction Center (HPC) continues to ramp up for Texas. Values from 1-4 inches look likely now with a solid 1-2 inches in south central Texas. Seen some parallels in the NWSFO AFDs to the event on Sept. 13-14. That would be great!

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weather_novice wrote:This rain is bad news! I have a 100mile bicycle race on Sat at 8am! Does anyone here think the main event will be Friday night and the rain will be gone by Saturday morning?

Portastorm wrote:

weather_novice wrote:This rain is bad news! I have a 100mile bicycle race on Sat at 8am! Does anyone here think the main event will be Friday night and the rain will be gone by Saturday morning?

I know you were joking about the first five words of your post, right?! Right??!!

Seriously, if you're in the San Antonio area I doubt the rain will be gone by Saturday morning. Maybe Saturday evening.

Terri wrote:Looks like we may be in for a moderate rain event. I really hate seeing the ground out in my pastures with cracks.... I am ready! The only drawback.... Labs coming in with muddy feet... But I have a mop and pail for that!

weatherdude1108 wrote:Yeah, I have to drive to San Antonio this afternoon, and come back tomorrow morning. I'm more concerned about the drive back tomorrow morning. I'd much rather be viewing it from a house than a car. I'll let you know what I find out and see, while being careful of course.

FXUS64 KHGX 281516
AFDHGX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HOUSTON/GALVESTON TX
1016 AM CDT FRI SEP 28 2012

...LOCALLY HEAVY RAINFALL EXPECTED ACROSS A LARGE PART OF SOUTHEAST
TEXAS OVER THE WEEKEND AS A STORM SYSTEM SLOWLY MOVES THROUGH THE AREA...

.DISCUSSION...
SCATTERED SHOWERS AND THUNDERSTORMS NEAR THE COAST AND OFFSHORE THIS
MORNING WILL WORK THEIR WAY INLAND TODAY. SIMILAR TO YESTERDAY AFTERNOON/
EVENING...SOME SPOTS COULD SEE SOME LOCALLY HEAVY RAINFALL IN THE SLOWER
MOVING ACTIVITY. GREATER RISK FOR HEAVY RAINFALL COMES SATURDAY THROUGH
SUNDAY MORNING WHEN A STORM SYSTEM DEVELOPS AND MOVES ACROSS OUR AREA.
VERY HIGH PRECIPITABLE WATER VALUES (>2 INCHES) COUPLED WITH DECENT
CONVERGENCE AHEAD OF A STRENGTHENING SURFACE LOW IS EXPECTED TO HELP
TO PRODUCE EVENT RAINFALL TOTALS AVERAGING 1 TO 2 INCHES WITH ISOLATED
AMOUNTS EASILY REACHING 2 TO 4 INCHES. EVEN HIGHER AMOUNTS CANNOT BE
RULED OUT! WITH AREA GROUNDS POSSIBLY BECOMING WET AHEAD OF THE MAIN
STORM SYSTEM...A FLASH FLOOD WATCH IS PROBABLY GOING TO BE ISSUED WITH
THIS AFTERNOON`S PACKAGE. EXACTLY WHERE THE HIGHEST RAINFALL TOTALS
AND GREATEST FLASH FLOOD RISK ENDS UP IS STILL HARD TO PINPOINT DUE
TO DIFFERENT MODEL SOLUTIONS CONCERNING THE LOCATION AND STRENGTH OF
THIS SYSTEM...BUT CONFIDENCE CONTINUES TO INCREASE THAT LARGE PORTIONS
OF SOUTHEAST TEXAS WILL RECEIVE HEAVY RAINFALL WITH THIS EVENT.

HERE ARE SOME AREA RECORD RAINFALL TOTALS (AND YEAR SET) FOR THIS WEEKEND.

SATURDAY (9/29) SUNDAY (9/30)

CLL - 1.84" (1921) 1.85" (1927)
IAH - 2.51" (1985) 2.55" (1913)
HOU - 1.70" (1985) 1.15" (1934)
GLS - 2.12" (1921) 4.33" (1945)