Good Morning,

With the sudden development of Tropical Storm Hermine the forecast for this week has changed substantially. What would have only been a minor heavy rain event has escalated into something of a more serious matter.

Tropical Storm Hermine made landfall just south of the Texas/Mexico border earlier tonight with winds of 65 MPH. The system was rapidly developing just prior to its landfall and it is quite likely it would have attained hurricane status had it even had another two hours over water. However now that landfall has been made my focus shifts to the possible impacts this storm will bring to North Texas. As I said above had this system not developed into a tropical cyclone we would have just had to deal with the deep moisture. In fact that’s not much different then the situation we spend in most of June and July, a lot of moisture without a specific focus to initiate convection. Obviously now that we have a tropical cyclone, that will weaken but maintain some low level wind fields, that forecast has changed.

The above graphic depicts the 11 PM forecasted track for Hermine. It is important not to focus on the center of the track as by the time the storm moves inland the effects of the storm will be felt across the entire cone of uncertainty. However there is still some importance where the center ends up. As is normal with a tropical cyclone there is a ‘clean’ side and a ‘dirty’ side. To the west of the circulation there is subsidence, keeping rain down to a minimum is most cases. To the east of the center there is the healthy tropical moisture, combined with the cyclone’s lift widespread convection often forms. With the extremely moist environment rain rates often approach extreme levels (3-4 inches per hour).  While not usually a constant rain, with enough tropical rain bands the rain total eventually add up and prove to be a problem. Right now is seems possible that parts of North Texas (areas along and west of I-35 pose the highest risk) will receive at least 3 inches of rain, with isolated total over 7 inches possible. Now three inches falling within a couple day time-frame does usually cause a flooding problem, but these rain totals could occur very quickly, thus not allowing enough time for the city infrastructure to keep up. This could cause potentially significant urban flooding if the rain falls quickly enough. If you live near a flood-prone area it will be crucial to monitor for any warnings that come out as rain rates will be extremely high.

While there is a high likelihood of flooding in areas, there is also the possibility of tornadic activity with this system. Shown below are the 850 MB wind charts from the 0Z GFS and NAM for 1 PM Wednesday.

While there are some subtle differences in the two models regarding the location of the highest winds, they are both in agreement that south to southwest winds of at least 40 knots will overspread North Texas from Tuesday Evening into Wednesday Afternoon. Although completely tropical in nature, these wind fields are more like systems you would see in the winter time in terms of their strength. Because the wind fields are so strong they can often help make up for a lack of instability, but some instability is still needed. In tropical environments such as this because their is often widespread convection limiting surface heating there is not enough instability for cells to maintain low level mesocyclones for a long enough period of time to produce a tornado.

The above two graphics are the Relative Humidity levels at 700 MB and 500 MB. The reason I’m displaying this is because the GFS, which the graphics are taken from, is showing a dry-punch coming in at 500 MB. This dry-punch would allow some instability to develop (shown below). Should this dry-punch occur and instability was able to develop then it would be likely that several tropical tornadoes from mini-supercells would occur east of the center. Right now the track of the low pressure puts North Texas in a very favorable position for tropical tornadoes, should enough instability develop. The forecaster on duty tonight at the National Weather Service noted in his evening discussion that Hermine could allow the highest tornado threat in ten years (from a tropical system) to occur Wednesday. Although tropical tornadoes are not typically long lived or significant, they can still produce damage. In this type of environment, where the shear is very high, it would be possible that a number of these tropical tornadoes could occur. Thus this would be a good time for you to practice your tornado safety plan, and if you do not have one already, to purchase and set up your NOAA All Hazards Radio. Tornadoes that form in this environment form very quickly and the lead time is often very low, thus it is essential you be ready to act if a warning is issued for your area.

The GFS, which seems to have a better handle on Hermine, is showing over 1,000 J/Kg of Surface Based CAPE developing by Wednesday Afternoon. If this were to materialize it would be likely that several tornadoes would occur across North Texas, already being underway by the noon hour. This situation is different then the typical tornado events that wait until late afternoon or evening. The storms, which would be located in the tropical rain-bands might be going tornadic by the noon hour should enough instability develop. Please stay tuned for the latest on this dangerous flood/severe weather event.

Here is the evening discussion posted by a forecaster at the National Weather Service in Fort Worth, Tx.

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE FORT WORTH TX
900 PM CDT MON SEP 6 2010

.UPDATE...
SOME MINOR UPDATES HAVE BEEN MADE TO THE FORECAST. WILL REMOVE
EVENING POPS AND LOWER THE POPS AFTER MIDNIGHT A BIT WITH MOST
ACTIVITY EXPECTED TO REMAIN SOUTH OF THE REGION. OTHERWISE...DID
MAKE SOME ADJUSTMENTS TO LOWS TOMORROW NIGHT TO RAISE THEM BASED
ON THE MID 70 DEWPOINTS EXPECTED TO ARRIVE HERE...AND ALSO LOWERED
HIGHS ON WEDNESDAY DUE TO EXPECTED CLOUD COVER AND RAINFALL.

SURFACE WINDS WILL DIMINISH WITH HERMINE AS IT MOVES INLAND...BUT
ALL INDICATIONS ARE THAT WINDS JUST ABOVE THE SURFACE WILL TAKE
SOME TIME TO SPIN DOWN AS IS USUALLY THE CASE WITH LAND FALLING
TROPICAL CYCLONES. THE HEAVY RAIN SHOWERS AND ISOLATED STORMS WILL
HAVE THE POTENTIAL TO MIX DOWN SOME OF THESE WINDS ALOFT...WITH
A LOW THREAT OF SEVERE WIND GUSTS POSSIBLE.

IN ADDITION NORTH TEXAS WILL BE IN A FAVORED REGION FOR TORNADOES
WITH HERMINE TAKING A TRACK THAT IS PERHAPS THE MOST FAVORABLE
TRACK FOR TROPICAL CYCLONE TORNADOES HERE OF ANY SYSTEM SINCE
FRANCES IN 1998. MOST MODELS ARE SHOWING LOW LEVEL SHEAR VALUES
STRONG ENOUGH TO SUPPORT TORNADOES TUESDAY AFTERNOON OVER THE
SOUTHERN ZONES AND TUESDAY NIGHT AND WEDNESDAY OVER MOST OF THE
AREA. THE MAIN LIMITING FACTOR FOR TORNADOES WITH TROPICAL SYSTEMS
IS A LACK OF LOW LEVEL INSTABILITY AND UPDRAFTS ARE THEN TOO WEAK
TO STRETCH THE AVAILABLE VORTICITY. THUS TORNADIC ACTIVITY THIS
FAR INLAND IS USUALLY DEPENDENT ON DIURNAL HEATING FOR THIS
INSTABILITY...BUT WIDESPREAD CLOUDS/RAIN OFTEN LIMIT THIS EFFECT
AS WELL. WE WILL CLOSELY WATCH INSTABILITY VALUES TOMORROW
AFTERNOON AND AGAIN ON WEDNESDAY TO DETERMINE WHETHER THE TORNADO
THREAT WILL ACTUALLY MATERIALIZE. ALSO RESEARCH SHOWS THAT
TROPICAL SYSTEMS THAT INGEST MID LEVEL DRY AIR HAVE A GREATER
FREQUENCY OF TORNADOES. CURRENTLY THERE IS A LARGE RESERVOIR OF
DRY AIR ALOFT OVER WEST AND NORTH TEXAS AND IT IS UNCLEAR HOW
THIS WILL INTERACT WITH HERMINE...IF AT ALL. AT THIS TIME THE
TORNADO THREAT IS NOT A GIVEN...BUT CERTAINLY HIGH ENOUGH TO
MENTION IN THE VARIOUS PRODUCTS AS A VALID THREAT AND WARRANTS
CLOSE MONITORING DURING THE NEXT COUPLE DAYS.