Ph.D. Student - Earth System Science (UC Irvine), B.Sc. - Atmospheric Sciences (Cornell University)
By: Zachary Labe , 11:42 PM GMT on May 10, 2008
"Thoughts of Flood Threat of May 11-13"
Good Saturday evening!!! As mentioned yesterday another storm is on the heals of the past storm that moved through the state. After a good soaking of rain in many areas and after a short wave moved through early this morning many areas in the west have had 2inches of rain in the last 2days. Interestingly enough a mesoscale complex formed last night over the Laural Highlands with heavy, heavy rain and embedded thunderstorms. Some radar estimates of rain approached 3inches in Somerset County. Now onto the next storm and here is another specially formatted blog for floods. And our next threat will last from May 11-13 with the highest threat area in the Laural Highlands. A low pressure coming out of the midwest will be moving through Maryland and causing a secondary low pressure to form. A fully detailed storm track explanation can be found below. This secondary low pressure will have a strong southeasterly component that will bring moisture up the eastern slopes of the Appalachians. Rain rates will be quite heavy at times as Atlantic and Gulf moisture combine. Temperatures will be pretty cool with low cloud ceilings at 1000ft so instibility looks pretty low. But with all the energy within this storm a few embedded low-topped thunderstorms cannot be ruled out especially in the southeast. Some coastal flooding may also occur in the coastal plain region of Pennsylvania near Philadelphia as easterly winds push surges up a few feet above normal. Winds will become breezy also on Monday but should not be too much of a threat. Again the highest area of flood threat is in the Laural Highlands where even flash flood risks are at a measly 1.5inches in 12hr period. The number will be doubled with this storm so flooding does look likely. A full flood risk map by myself is issued below. This is a complex storm system with many variables involved. Also it is a quite intense coastal storm for May standards. If only this was January! My biggest question is how strong the blocking high to the north will be and I am wondering if the models are in Spring mode and possibly underestimated the high's intensity. Anyway as usuall updates will be provided leading up to the event, during the event, and after the event. So stay tuned!!!
Recap on Last Rain Storm...
Well a little late, but here is a recap of the last rain storm that lasted from May 8-9. As the cold front moved in across the region a warm front also began to move northward. Warm and moist air flowed northward into a narrow alley just east of the Appalachian Mountains in western parts of the state of Virginia. Supercellular thunderstorms began to form stretching from North Carolina to Virginia. I must admit that those storms were the most intense I have seen indicated on radar in the Middle Atlantic region in quite a long time. Several tornadoes occured in the region with damage in a few areas. Also there was damage from straightline winds outside the Washington DC metro area. All of this energy stole energy from the main low pressure system that moved through Maryland and then up the eastern seaboard resulting in less rain than expected. Also a stronger blocking high formed to the north surpressing some of the precipitation to the south. Overall I was not too pleased with my forecast. My bust area was in central Pennsylvania with western Pennsylvania and eastern Pennsylvania with a pretty good forecast. Below is a verification map with total rain amounts against my forecast map. I will continue to issue these verification maps after each storm. And for this current ongoing storm I will issue wind and rain verification maps.
(Courtesy of NOAA)
(Courtesy of Penn State Meteo.)
An upper-level short wave is dropping down into the Midwest accompanied by a cold front. Behind the front will be again cooler air. The wave will move southeast into the upper Mississippi/Tennessee Valley and strengthen into a strong low pressure system. Pressures are expected to reach as low as 996mb. The low will gradually move east through Indiana and Ohio. To the south of the low strong convection is going to form with a widespread severe weather outbreak. For Saturday night the outbreak will be widespread across the southern Mississippi Valley. It is a very complex situation in which the warm front extends from Arkansas to Georgia. The dry line in Texas. The low pressure in Illinois. And the cold front in Kansas. All of these boundaries will meet across the southern Mississippi Valley where supercell thunderstorms will form. As the night progresses the storms will organize into a derecho line of thunderstorms with winds as the main threat up to 90mph straight line winds. This derecho will continue across the entire southeast. I expect damage reports well over 200 by the time the event is over. Then as the low moves east for Sunday the outbreak will shift into the Georgia and South Carolina area. Severe weather will be about as far north as the southern Virginia region. Isolated tornadoes will be found in this day of the outbreak, but wind damage from straight line winds should be the main threat. The low pressure will move then southeast across the Ohio River and gradually weaken. Though moisture will be well out ahead of the storm. Then a shift in energy will create a secondary low pressure of the Delmarva. Strong S/W energy will push a southeast wind up the eastern slopes of the Appalachians. Heavy rain will be common across east-central Pennsylvania into Maryland and northern Virginia. But to the north will be a strong surface high pressure acting as a blocking mechanism. This will prevent the heavy rain from getting to far north. Gradually by Monday night the low will move east and the cold front will filter in across the region. A snowflake cannot be ruled out after the front goes through and showers are left behind. The flakes though will have no impact and will be found at elevations above 3000ft. Overall this is a very complex storm system.
(Courtesy of NOAA)
"12hr Estimated Precipitation"
(Courtesy of NOAA)
"Forecast from Hydrometeorological Prediction Center"
QUANTITATIVE PRECIPITATION FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
559 PM EDT SUN MAY 11 2008
MAIN DIFF BETWEEN THE LATEST GUID AND PRVS GUID IS A SLGTLY MORE
SRN TRACK TO THE VRY STG/UNSEASONBLY DEEP MIDLVL LOW AS IT MOVS
THRU THE MID ATLC AND OFFSHORE. ALL GUID HAS SHOWN A TREND TO
INCRS PCPN AMTS...WHICH SEEMS TO BE LINKED TO THE MAIN BAND OF
PCPN BCMG NRLY STNRY AS IT PIVOTS ARND THE MIDLVL SYS. STG MIDLVL
HGT FALLS/UPR DIFL AND LEFT EXIT REGION DVRG WL PROMOTE A BROAD
REGION OF LIFT. THE LOLVL FLOW IS ALRDY REACTG TO THE APRCHG
MIDLVL SYS WITH A RAPID INCRS OFF THE ATLC THAT WL DRAW IN DEEP
MSTR AND SUPPORT SOME ELEV INSTBLTY. A PW PLUME FM THE GULF AND
ATLC ARE XPCTD TO MERGE WITH PWS ARND ONE INCH FEEDG INTO THE
REGION. THESE VALUES ARE NR NRML. 8H-7H FLOW OF 30-50 KTS WL
SUPPORT MOD/STG MSTR FLUX/THETA-E ADV WHILE STG LOLVL FLOW
SUPPORTS STG UPSLOPE INTO HIER ELEVS. MSTR FLUX ANOMALIES ARE FCST
TO BE ARND TWO STDS ABV NRML TNGT ACRS THE REGION. XPCT THE STG
DYNAMICS TO COMPENSATE FOR THE NR NRML MSTR LVLS AND THINK THIS IS
THE REASON FOR THE WETTER MDL SOLUS. XPCT A LRG REGION OF LGT/MOD
PCPN AMTS WITH A SWATH OF 1-3 INCH RNFL ALNG THE MASON-DIXON LN
WITH ISOLD HVIER AMTS OVR THE FAVORED UPSLOPE REGIONS
"HPC Forecast 3-day total QPF map"
Well lets take a look at the current models. Below are the GFS and NAM models which have total precipitation through 60 hours posted below as of the 12z model run. Also below is the simulated radar from the NMM model for 24 hours out from that 12z model run also. Models have been back and forth with positions of low centers. Recently the NAM has trended southeast and is now considered the outlier. The 12z GFS is in pretty good agreement with the 12z EURO model. It does appear though that the storm is moving at a slower intensity than what the models indicate on their maps, this may make matters worse for heavy rain falling. Overall though agreement in models looks pretty good. Only concern is the strength of the blocking high to the north of the lows. Also now appears that the secondary low pressure is forming farther south in the Carolinas instead of the Delmarva, again this may prolong the entire event. Stay tuned for further updates on short term models.
"GFS model total precipitation"
"NAM model total precipitation"
"NMM model future simulated radar at height of storm"
"12hr Flash Flood Guidance"
(Courtesy of NOAA)
Alrighty below is a look at all of my forecasts divided into subgroups...
Southeast winds will be a threat Sunday night across many of the ridge tops. High Wind Warnings are in affect for parts of the Laural Highlands. Already wind gusts over 50mph have been reported along with damage reports coming in. Trees and wires have been reported down in the following counties... Westmoreland, Fayette, and Somerset Counties. Winds from the southeast will begin to die down later Sunday evening. But through Monday night breezy conditions will persist, especially near the Atlantic Coastal Plain Region in Pennsylvania near Philadelphia. A brisk low-level jet with about 65knots winds. The strongest of winds will not mix down to the surface, but still it will be breezy with an occasional 45mph gust as dynamic conditions during the heavy rain mix some higher gusts from the jet down to the surface. Overall the worst of the wind should be gone by early Monday morning. A wind gust forecast verification map will be issued after the storm is over.
"My Forecast Wind Map"
The low pressure is continuing to transfer energy to a secondary low pressure that should form somewhere along the Atlantic Coast. Rainfall will begin to pivot on radars and form as east to west type of rain band as the night progresses. Atlantic moisture will hit its max with PWATS several deviations above normal. Already that is occuring in Virginia and southern Maryland where up to 2.5inches has already fallen. The heaviest of rain in Pennsylvania should fall in the blue area indicated below in my rain map where over 3inches of rain may fall. A verification map will be issued at the end of the storm for final rain totals against my forecast totals.
"My Forecast Total Rain Map"
Overall flood threat should be minor for most waterways. Though in the Laural Highlands it could be in the moderate stage on some smaller streams after up to three inches of rain in the last three days and another three more inches to come. Urban and small stream flooding will be the primary threats along with some coastal flooding. See more details on coastal flooding below. Major rivers should see level rises but only probably to caution stage for most river level reading locations. Below is my flood risk map area with the Laural Highlands most at risk for some sort of flooding. Flood watches are out for many areas across southern Pennsylvania. A verification map will be issued at the end of the storm for flood reports against my forecast map.
"My Flood Risk Map"
Coastal Flooding Threat...
As the strengthening coastal low pressure moves off the east coast gale force winds will develop from the northeast pushing water farther inland than normal. At the high tides in Philadelphia which is 8:22am, heavy rain will already have occured and the low pressure will gain even more strength. Water levels along the Delaware Bay will approach moderate flood levels in a few locations. A coastal flood warning is in affect for Philadelphia and Delaware Counties in Pennsylvania. Beaware while commuting Monday morning as the flooding will be reaching its peak during those key commuting hours.
Instibility is very low as cloud bases continue to lower, but still shear levels are high along with southern convection from Virginia streaming northward may cause a few thunderstorms in southern Pennsylvania. Nothing too significant or severe looks likely though. But any thunderstorms will continue to enhance the heavy rain.
This section is sort of for laughs. But anyway yes snow may occur in the highest of elevations during Monday night. As the secondary low pulls away 850s will cool below freezing. Also surface temperatures that night will be in the mid 30s in some areas. Left-over moisture will create a few rain/snow showers in the northwest and northeast mountains that have elevations of over 2600ft. So do not be surprised to see a few snowflakes in the mountains for Monday night.
As far as conditions are concerned they appear to be in the IFR range across many areas tonight with patchy areas of LIFR in the heaviest of rain. Cloud bases will lowers of the common orographic regions with ceilings at around 1000ft in some areas. Fog may be a threat too as dewpoints and temperatures have a limited spread. Wind shear levels are very high with 40knot winds only a few thousand feet above the surface. As the low pulls away these winds will mix down to the surface. Overall if traveling expect major delays with cancellations over the next 24hours. Drier and calmer conditions will move in for Tuesday.
"Conclusion on Flood Threat"
For many people it is a wonderful mother's day. But for the thousands of people in the southeast it is a day of the renewal of their lives and the start of the rebuilding process. Over 20 people lost their lives overnight last night as a derecho marched across many states. Over 350 severe weather reports occured yesterday and again there are more today. Our thoughts and prayers let them be with the victims of the deadly tornadoes. Anyway onto our state's weather. Overall I am not expecting too widespread of flooding. But none the less there will be areas of flooding especially in the Laural Highlands. This is a beautiful mid-Spring nor'easter that will cause heavy rain and high winds to affect much of the state of Pennslvania and entire Middle Atlantic region. The New England states should not have that much of an impact. Rain is already beginning to fall here north of Harrisburg and will last for over another 24 hours. For true weather lovers, like myself, this will be an absolutely fascinating storm to follow making forecasting all the more fun. I hope to provide some tidbits on coastal storms like this and maybe add some interesting forecasting techniques that I like to use. Again a recap will be issued after the storm is over along with 2 verification maps for wind gusts and rain totals against my forecasts. I feel very confident on my maps I made yesterday and I am going to make no changes to them. Let us all monitor and see what happens with this unsuall May coastal storm. Have a great day!!!
COMING AFTER THE STORM!!!
"Here north of Harrisburg 2008 statistics"
(Severe Weather Stats...)
Severe Thunderstorm Watches- 0
Severe Thunderstorm Warnings- 0
Tornado Watches- 0
Tornado Warnings- 0
Total Thunderstorms- 9
Flood Watches- 4
Flood Warnings- 3
Monthly Precipitation- 1.20inches
Yearly Precipitation- 14.86inches
Heat Advisories- 0
Excessive Heat Warnings- 0
90degree days- 0
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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