Ph.D. Student - Earth System Science (UC Irvine), B.Sc. - Atmospheric Sciences (Cornell University)
By: Zachary Labe , 1:45 AM GMT on August 27, 2009
Every now and then I do enjoy straying away from a set organized blog and enjoy giving a more meteorological approach for a weather system coming towards the region. Just recently I issued that blog last weekend about the Predecessor Rain Event (PRE) and a severe weather forecast which gave more explanations on how I came about my forecast which ended up being one of my best severe weather calls if I do say so myself, hahaha. Anyways I thought to cover the approaching weather system and Tropical Storm Danny I might as well issue a short term blog that looks at the forecast in the immediate term. Also Sunday I will put out my Summer Outlook review, which is not going to pretty. Then for next weekend I will be debuting my Winter Outlook 2009-2010, which should make some happy and others maybe not so much; you will just have to wait and see. In any case a lot of blogs coming up. This one is featuring an overrunning precipitation event for much of the Middle Atlantic during the same time Tropical Storm Danny moves up along the coast. First I will take a look at the overrunning precipitation event and show how this will give us a taste of October stratiform rain events. Current water vapor loops shows some interesting features as a cold front located over the Middle Atlantic pushes southward drawing in drier and cooler air for Thursday. Lack of any trigger and moisture aloft will mainly cause a dry frontal passage.
Fig 1.0- A few important features to note on the water vapor loop. First off across the Midwest lies the next cold front and associated low pressure with a steep elongated trough that is causing near record lows over the northern Midwest with lows down in the 30s! South of the front is that pesky upper level low over the southeast near Alabama and Louisiana which is still shearing apart Danny on the eastern side. This dry air and associated upper level low will continue to weaken and moves West-southwest. These are the players in the next couple of days that will factor into our weather
By Thursday evening things begin to turn downhill from Maryland on through Pennsylvania as isentropic lift (when warm air lifts over a cooler airmass) touches off precipitation along the quasi-stationary boundary that stalls near central to northern Maryland. Cool air at the surface with a north wind, but aloft a southerly wind with an influx of tropical air will form an overrunning precipitation event for the upcoming days.
Fig 2.0- Current 18z GFS shows the 700mb RH values for Thursday night as precipitate waters rise to a few deviations above normal likely about 1.5inches. Keep in mind true tropical air has PWATs near 2inches+, but this overrunning event will be fighting some drier air from a weak low pressure system.
Meanwhile across the western United States a strong southwest ridge is forming and the several deviations below normal upper level trough is moving across areas from the Midwest on eastward. This pattern is very favorable for unseasonably cool air to transfer from Canada through the United States
Fig 3.0- 18z GFS looking out towards Sunday afternoon shows how cool in the incoming airmass actually is with H85s dropping to well below normal values for this time of year below 10C. Also note the 0C is not too far away from the Northern Great Lakes there in Canada. Current GFS meteograms drop lows down in the 40s for areas as far south as Harrisburg several times next week with dry dewpoints down in the low 40s. Link. This just goes to show the anomalous trough approaching that will help steer Tropical Storm Danny.
So all this put together means a wet and cool period is upon us despite what influence Tropical Storm Danny may have on the region. Thursday evening through Saturday several waves of low pressure will ride along the quasi-stationary boundary (stalled front). This will allow periods of damp conditions to occur with generally light to moderate steady rain periods especially for the southern half of Pennsylvania on southward through Maryland and Delaware. Convective chances remain slim with a relatively stable airmass although a few elevated thunderstorms are possible. Also with the overrunning event low clouds and fog will be common with the temperature contrast similar to what occurs in Fall and winter with low clouds featuring dense fog over the mountains with ceilings as low as 300ft. This will bring IFR conditions to the airfields especially south of I-80 this week. If you are unfamiliar with my use of VFR, MVFR, IFR, LIFR terms, that are acronyms for aviation use. For instance IFR stands for instrumental flight rules. VFR conditions are for clear days with low aviation threats with conditions deteriorating conditions to LIFR which stands for limited instrumental flight rules which occurs during low visibility or heavy weather. Anyways this week will be common for low visibility with fog and drizzle with visibilities down around 1-3mi. Timing is a bit questionable for Thursday night through Friday night with the waves of precipitation, but Friday should feature the majority of the overrunning precipitation with cool temperatures and an easterly 90degree wind. On occasion, especially for lake effect snow, you will see me reference degrees off the mathematic circle degrees 0-360. This use is common in meteorology and allows for more accuracy than saying a west-southwest wind. A west-southwest wind could mean anywhere from a 200degree to 220degrees. So therefore saying a 210degree wind is much more precise. Anyways I am getting sidetracked... For those looking at a weather map without any isothermals, it would look very similar to an overrunning snow event. We have a 1204 anticyclone (high pressure) over southern Canada in Quebec with a stationary front to our south allowing precipitation to ride up along it with drizzle intermittent thanks to a temperature contrast.
A cold front bring the upper level trough eastward will be approaching the region by Saturday allowing for the winds to shift out of the southwest eroding much of the marine layer for areas west of the Susquehanna River. As Tropical Storm Danny begins to move up the coast a more tropical airmass will encompass the region with higher dewpoints and slightly more unstable air.
Fig 4.0- The 15z SREF model, which is a higher resolution for short term forecasts shows rising CAPE values at the surface in excess of 500 j/kg for areas south of the Pennsylvania turnpike through Maryland and Delaware. For those who have been reading my blog and are afraid to ask, CAPE stands for convective available potential energy basically measuring the buoyancy in the atmosphere. For the science buffs out there and mathematical wizards here is the official American Meteorological Society definition and formula...
convective available potential energy—(Abbreviated CAPE.) The maximum energy available to an ascending parcel, according to parcel theory. On a thermodynamic diagram this is called positive area, and can be seen as the region between the lifted parcel process curve and the environmental sounding, from the parcel's level of free convection to its level of neutral buoyancy. It may be defined as
where αe is the environmental specific volume profile, αp is the specific volume of a parcel moving upward moist-adiabatically from the level of free convection, pf is the pressure at the level of free convection, and pn is the pressure at the level of neutral buoyancy. The value depends on whether the moist-adiabatic process is considered reversible or irreversible (conventionally irreversible) and whether the latent heat of freezing is considered (conventionally not). Compare convective inhibition.
Anyways as we enter Saturday we push the marine layer back over the Atlantic with a more tropical flow with scattered showers and thunderstorms triggered from several boundaries including the cold front to the west, the warm front rising over the region, and moisture from any Predecessor Rainfall Event over eastern and southern areas. As tropical systems move northward in latitude the increase in forward momentum as they enter a faster jet stream pattern in prevailing westerlies. So by morning Tropical Storm Danny will be approaching the coordinates 35N 75W, which is right off the coast of Hatteras. Intensity forecasts are a bit difficult with Danny as the influence of that Upper Level low causing dry air over the open low level circulation of Danny. But the ULL will weaken with time by Friday afternoon. Danny will enter an area of warm gulf waters near 28C and slightly strengthen for an optimum period of 12hrs of so before undergoing shear from the approach of the anomalous upper level trough. Danny will begin marching shifting from WNW Friday to NW Friday evening to N Saturday morning to NNE Saturday evening to NE Sunday morning. My official forecast track is for the following coordinates... 35N 75W...39N 73W... 41N 70.5W...43N 68W. Basically if you don't feel liking looking up the coordinates it is a few miles off of Hatteras...75 miles off of Atlantic City curving and cruising across Cape Cod making its first landfall. As far as intensity Danny has the potential to approach a strong Tropical storm or minimal hurricane with winds up to 100mph. But by Sunday approaching a Cape Cod landfall this system will potentially transition quickly to a cold core system which is more of a hybrid than purely tropical. This should prevent Danny from strengthening further more than a category 1 hurricane.
A quick look at the Wednesday evening models shows a bit more of a shift eastward in the foreign models including the ECMWF and the GGEM. The GFS is the farther outlier outside of a few hurricane models basically not phasing this system with the steering currents of the upper level trough and therefore keeping the intensity down and keeping this farther east. Keep in mind GFS data is initiated in the hurricane models such as the GFDL and HWRF, so they typically have semi similar paths. So with the forecast path would probably influence several coastal regions in the Middle Atlantic. Here is the current Wave Watch III prediction.
Fig 5.0- The current Wednesday evening Wave Watch III model initiated off the GFS data keeps the highest wave heights offshore generally around 6-10ft with lower waves closer to the coast.
The center of Danny will pass to the east of all coastal locations in the Middle Atlantic which is a positive as the western quadrant typically as lower wave heights, storm surge values, and lower wind speeds, but heavier rain. The eastern quadrant contains the fury of the storm, but with rain more organized into feeder bands. For areas across the Chesapeake and Delaware Bays I would estimate advisories being issued for the waterways starting late Friday as the system approaches the region. Winds for eastern buoys may approach 40knots for max speeds with gusts up to 45knots for a very brief period.
For another reference the NWS threshold for gale force winds is 34-47knots. Winds will be strong, but not enough to cause much more than minimal damage. Wave heights for the Bay will be a few feet above wave crests after Bill's passage. Storm surge will likely be minimal also around 1-3ft for the Bay especially towards the mouth of the Delaware and Chesapeake. Towards the coast with Danny about 85 nautical miles offshore (100miles) off the coast of KACY (Atlantic City). This will cause a bit rougher conditions over the Atlantic coasts of Delaware and Maryland with winds likely approach 50knots for Ocean City piers and such for a short period. The baroclonic pressure gradient is not overly impressive in the western quadrant of Danny as it passes offshore. Storm surge along the Atlantic coast in the same areas will again be minimal around 1-4ft with waves approach 5-10ft, likely higher depending on strength of Danny. As the storm moves northeastward likely moving towards Cape Cod, conditions will begin to improve over the Middle Atlantic coastal waters. Keep in mind also for beach enthusiasts again that there will be red flag warning conditions for rip currents, and it is not a good idea to be in the water from Friday through Sunday. We all saw what happened in the waters during Bill's offshore passage. The last marine threat other than heavy rain is the threat of waterspouts which are possible as the feeder bands from Danny rotate through the region, with waterspouts developing potentially with little to no warning.
This brings me up to my final point of QPF totals for directly related to Danny. The rain shield remains in question in the western extent. It does appear extreme eastern Pennsylvania, eastern Maryland, and Delaware will feel some of the effects of the heavy rain from this system. PWATs will rise to near 2inches for near the coastline allowing for tropical downpours and rainfall rates near 2inches per hour for some areas. Rainfall totals will likely be highest towards the coast with some areas seeing up to 3inches of rain or more.
Fig 6.0- HPC continues to show heavy rain over the region with the core of the heaviest rain over the open waters over the Atlantic. These are total QPF from Danny in the map above.
Finally I would like to list my concerns. I know this all sounds like a final forecast and that Danny is approaching in 24hours or so, but that is not true. And many regulars here on this blog know I do not issue set in stone forecasts until 24hours before a system. This is a general outline on how the synoptic setup currently looks. In fact Danny is several days away from approaching the Middle Atlantic or the United States for that matter. This means that a lot of changes are possible in the forecast, especially the track. After analyzing today it is interesting to note the center of circulation still is open and being sheared apart with little convection surrounding it, generally to the east. There are also subtle signs of models shifting a bit more to the east which would lessen the impact on the United States. The coming days are key to the final forecast and the above discussion is just giving a general heads-up for this weekend in terms of weather, and I know many changes are likely to occur. The best bet is to stay tuned to this quickly changing event, but for now I hoped I highlighted most people's concerns and gave my best effort on how the situation currently looks. I will be posting throughout the coming days on any updates that occur. By the way I am plotting the track of Danny, yes pencil and paper; some us like the old fashioned method, hahaha, so if anyone needs coordinate points and such, just ask! Stay tuned and have a wonderful weekend!
Fig 7.0- Current satellite of Tropical Storm Danny
"Here 10mi Northeast of Harrisburg, PA 2009 Statistics"
(Severe Weather Stats...)
Severe Thunderstorm Watches- 8
Severe Thunderstorm Warnings- 9
Tornado Watches- 0
Tornado Warnings- 0
Total Thunderstorms- 35
Flood Watches- 4
Flood Warnings- 1
Monthly Precipitation- 5.16inches
Yearly Precipitation- 28.83inches
Heat Advisories- 2
Excessive Heat Warnings- 0
90degree Days- 9
Highest Temperature- 95degrees
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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