Ph.D. Student - Earth System Science (UC Irvine), B.Sc. - Atmospheric Sciences (Cornell University)
By: Zachary Labe , 8:15 PM GMT on September 05, 2008
Tropical Storm Hanna...
(Courtesy of Penn State EWall)
For me tropical season always brings lots of headaches, but yet the thrill of excitement when a storm heads my direction. It has been a pretty long time since a significant tropical storm has moved my direction. The last major storm I remember was Hurricane Isabel which headed up the coast bringing winds gusting over 60mph here in Harrisburg. So now, September 2008, we have Hanna headed our way. This blog is a different format than usuall due to the fact that a tropical system has not impacted our area this year. By the time Hanna reaches our area it will maintain tropical storm force with winds sustained near 50mph near the center of circulation. But Hanna has a very large circulation which will spread strong wind gusts widespread over the region. Hanna will also cause some very heavy rain as tropical moisture overspreads the region with high PWAT anomalies. Some areas may recieve up to 8inches of rain across southeastern Pennsylvania. The ground is very dry and this may cause flash flooding as the ground will not be immediantly ready to suck in the rainfall. I will talk all about the immediant impacts on the region in the sections below.
Track of Hanna...
(Courtesy of NOAA)
(Courtesy of NOAA)
Rainfall- The largest impact on the state of Pennsylvania from Hanna will most likely be rainfall. A narrow band of very high anomaly PWATs is streaming northward across the eastern half of Pennsylvania. Hanna has had a very subtropical look to it with most of the moisture confined to the western half of the circulation. A very large precipitation shield of light to moderate rain will overspread the region ahead of the circulation overnight late tonight and early tomorrow morning. The center of circulation will lift northward across the region with a large moisture shield to the west of the circulation. The center of Hanna I believe will track just to the east of the I-95 corridor. As the rain shield becomes heavier, rain rates may approach very high rates near 2inches per hour. Tropical moisture for a relatively unknown reason does not show up on doppler radar very well. It seems the radar has less DBZs than what really should be indicated. Rainfall amounts are going to exceed 3inches in many locations. There will be a sharp cutoff to the heavier precipitation and I think that will eye up just to the east of the central Pennsylvania line. Rainfall will be heaviest along the southeast piedmont region with isolated rainfall amounts nearing 9inches. But Hanna will be moving at a very fast pace so amounts more than that are not overally likely. Conditions have been very dry over the region lately and the ground has almost become like cement. Heavy tropical downpours will not be immediantly able to sponge into the ground, so they will rapidly run into urban areas and streams causing areas of flash flooding. River flooding is not likely due to recent low water levels. Rainfall should beginning to subside across the region by 8-9pm Saturday evening. Most all areas east of the Susquehanna River will see rainfall amounts over 1-2inches. Here is my rainfall map...
Wind- Wind will more of an impact than originally thought to be several days ago. It appears as if the center of circulation will travel up through North Carolina up through eastern Virginia then up through the Delmarva. The tropical storm force wind shield will be pretty large with winds sustained over tropical storm force wind as far west as the Lehigh Valley. Areas to the west of the Lehigh Valley but east of the Susquehanna River will still see tropical storm force wind gusts, but not likely to see sustained winds over 39mph. Winds will gust up to near 60-65mph across far eastern Pennsylvania. Winds of this magnitude are capable of downing trees and powerlines. This evening I would suggest making sure all objects are strapped down so they do not get blow around. The area with the highest threat of wind is around the Philadelphia metro area. Currently they are under an inland tropical storm force wind warning. Elsewhere areas may see winds gust up to 50mph. Here is my current wind map...
Storm Surge- Pennsylvania off course does not have any ocean coastlines, but for those with concerns off the Chesapeake it appears there will be a 2-3ft water rise. The highest water level rises will occur around midday Saturday as the circulation of Hanna approaches the bay region during coastal high tides. Here is a list of coastal high tides along the Chesapeake Bay...
HAVRE DE GRACE...2:42 PM SATURDAY
BOWLEY BAR...12:22 PM
FORT MCHENRY...11:31 AM
FELLS POINT...11:40 AM
CHESAPEAKE BEACH...8:39 AM
SOLOMONS ISLAND...6:46 AM
"Radar for Central Pennsylvania"
"Radar for Eastern Pennsylvania"
Severe Weather- I am not looking at too much of a severe weather threat with Hanna. Areas to the east of the center of circulation will be under the highest threat for severe weather. As with any landfalling tropical system, tornadoes are always a threat. Here is the latest from the SPC...
03Z OFFICIAL NHC FCST PLACES THE CENTER OF TC HANNA OVER THE SE CSTL
PLAINS OF NC AT 12Z...SRN NJ AT 00Z/07 AND THEN ACCELERATES IT NEWD
INTO THE GULF OF MAINE BY 12Z/07 AS AN EXTRATROPICAL CYCLONE. DEEP
MOIST ADIABATIC PROFILES AND 0-1KM SRH OF 300+ M2/S2 ALONG/E OF THE
TRACK OF THE CENTER WILL MAINTAIN THE THREAT OF BRIEF TORNADOES/
WATERSPOUTS THROUGH THE PD.
Current Surface Map...
(Courtesy of Penn State EWall)
Current Weather Map...
Looking at the current weather map we have a cold front moving quite slowly across eastern parts of Ohio. Much of Gustav's moisture with the front has fizzled out, so I am not expect much precipitation for western Pennsylvania. A southern low-level jet will nose its way in across Pennsylvania making for some strong winds aloft. A deep trough is currently moving through the Midwest with very dry upper and mid levels. A high pressure across the Atlantic is departing that area and moving east. This all is the perfect setup for a tropical storm to come up the coast and ride around along the trough.
Current Water Vapor Loop...
(Courtesy of Penn State EWall)
Computer model forecasts...
Looking at the computer models I am going with the 12z HWRF track. I believe this model has a great track record this tropical season and I believe it has a good hold on the forecast with Hanna. Looking at other models the NAM has the storm slightly farther west than other models, but the NAM has been not intiating the actual National Hurricane Center location for Hanna, so for right now I am throwing away the NAM. The GFS is a little farther east with this system. I think the GFS is just to far east in the track. Here is the 12z HWRF close up over Pennsylvania...
It should definitely be an exciting day on Saturday for many areas across the Middle Atlantic. Winds will gust over tropical storm force at times and we will all get a very nice beneficial rain which still could cause chaos with areas of flooding. I will be on most of Saturday to relay reports from my area. I checked my weather station and it looks all set. This forecast above was relatively short based on other forecasts I have made before, but I really did not want to make it too detailed for this event. This blog still is under construction through the weekend as changes come about. I look foward to hearing observations from across the region on Saturday. We are ready for you Hanna! Prepare for the worst, but hope for the best. Have a wonderful day!!!
Forecast Model Links
-NAM model 12z...Link
-GFS model 12z...Link
-NMM model 12z...Link
-SREF model 9z...Link
Severe Weather Links
-Atmospheric Soundings Skewt T charts...Link
-SPC Mesoscale Analysis Pages...Link
-Public Spotter Reports for State College NWS...Link
-Severe Weather Model Forecast indices...Link
-Severe Weather Parameter Definitions...Link
-Automated Pennsylvania Rainfall Recording Stations...Link
-Flash Flooding Guidance...Link
-HPC Forecasts for Excessive Rainfall...Link
-Hydrology Predictions for Lakes, Rivers, and Streams...Link
The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.
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