Unusual warm again and tropical trouble...
In a few short days the meteorological summer will be only a recent memory. It is already evident seasonal changes are quickly abrupt considering darkness arrives before 8pm allowing for shorter days. Nights are starting too cool off as the first 30s and 40s have appeared for many areas already. Morning fog is also a typical morning nuisance as common with the Fall season courtesy of differing land/water temperatures from radiational cooling. Summer 2010 will be defined as a very hot summer setting many records across the eastern seaboard. After the anomalous snowfall of February 2010, monthly temperature anomalies have been a constant above normal.
August- (+1.9F) Preliminary
Scores of high temperatures have been broken across the Northeast as temperatures soared into the 100s for the first time for many areas since 2002. For many, July 6 was the peak of the heat with most ASOS stations reporting temperatures of 100F or higher. My PWS even reached 101F, the highest since the station has been automated back in 2007.
Fig. 1- Unlike many heatwaves for the eastern US which derive out of the southwest, these abnormal thickness developed from a bubble of high pressure (heat dome) directly over the Middle Atlantic. Therefore the eastern US featured the highest temperatures and the Midwest avoided the heatwave. This was common for many of the heatwaves this summer, but this particular map above is thickness from July 6.
This abnormal warmth can likely be directly correlated to a very unusually warm winter in Canada. General jet stream wavelengths favor airmasses moving from Canada into the United States for most seasons because our trade winds blow west to east in this part of the hemisphere. Canada featured one of its warmest winters on record with a very low snow cover and depth nationwide. Alaska also featured a very mild winter with even only 22in of snow in Fairbanks, AK. Therefore there has been very little cool air in Canada to alleviate the abnormal warmth downstream in the United States. While a few backdoor cold fronts did occur sparking some severe weather, in general the progression of cold frontal passages through the Middle Atlantic was below normal. But the summer of 2010 has allowed for a bit of cooling in the Arctic despite continuing sea ice melting, including the opening of the Northwest Passage.
Fig. 2- Current sea ice levels remain similar to numbers at this current time last year.
As we wrap-up discussions on the abnormal warmth in the northern Middle Atlantic, another five day heatwave bears down on the east coast. Current H85 thermals progged from the GFS indicate a pocket of x>20C temperatures during the peak of the heat Wednesday and Thursday. While the availability of the heat begins to become more limited as we enter September, a few records remain in the upper 90s to 100F for this time period, so abnormal warmth is certainly not unprecidented. The upcoming period will be hot, but with dry humidity levels. PWATs aloft and H7 moisture anomalies will remain very low all week and the flow out of the southwest from the Gulf of Mexico remains interrupted. Therefore humidity levels will stay low allowing dewpoints to remain in the 50s and 60s. Therefore these dewpoints combined with temperatures in the low 90s will actually make it feel slightly cooler on the body. Heat indices will be in the upper 80s to 90F all week, despite a few locations that could see high temperatures top 95F especially towards Washington DC. These 90F days will just adding to the impressive of 90F highs this year which is already topping 30 here in Harrisburg. Current MOS output forecast highs for Washington DC for upcoming days...
The forecast remains simple for the Sunday through Thursday time frame for all areas.... sunshine with patchy morning fog with lows in the 50s, followed by increasing heat with highs near or slightly above 90F. Elevations above 2000ft will remain in the mid to upper 80s.
A dry cold front passes through Thursday night alleviating this abnormal warmth and probably making this 5-day heat wave, the last of its kind for another many months.
Fig. 3- Current GFS/ECMWF 10-day median thickness anomalies indicate possible troughing over the East Coast towards early to mid September. In fact at this point, it appears temperatures may be on the cool side towards Labor Day.
By the end of the month though I do expect September to probably range ~(+)1F above normal for most areas temperature wise with below normal precipitation.
Fig. 4- 12z ECMWF ensembles continue to indicate a negative west based NAO, but NAO affects as far as Northeast troughing usually do not directly correlate until October onward through winter. The NAO has relatively limited effects during the summer for boundary layer temperatures. For instance most of the summer has featured above normal temperatures despite a negative NAO most of the summer.
With ridging over most of the east coast (note the building of a possible southeast ridge already), most areas in the next two weeks will feature above normal temperatures with a possible quick reprieve of cooler temperatures around Labor Day. Also conditions look abnormally dry. Current 28 August 2010 12utc GFS only prints out .17in for KMDT in the 16-day QPF meteogram.
Finally while the boring weather pattern continues for the northern Middle Atlantic, the tropics begin to pose a possible threat to the east coast. Hurricane Danielle will continue to swirl swinging well offshore from Bermuda. Despite this the increasing surf will be common along the eastern seaboard and here in the northern Middle Atlantic along the Delaware and Maryland shorelines. WaveWatchIII model indicates swells on the order of 5-7ft for Atlantic coastlines in Maryland and Delaware. This will cause a bit of beach erosion and an increased rip current threat. The tropical system Earl is shortly behind Danielle and will come slightly closer to the East Coast, but a trough moving through the NOCONUS will aid in swinging the storm system likely between Bermuda and the United States. Recent GFS prognostics have occasionally hinted at a close proximity of Earl and areas such as Cape Cod, but I think the model is not swinging the trough through fast enough. At this time I think the main threats will be beach erosion and increased surf as this storm too, passes well offshore. Still though as anything with tropics, it remains highly uncertain. Finally 97L will likely be named Fiona in the coming days and will follow a track south of its formers. This will take it very close to the Antilles posing another tropical threat to those islands after Earl. During that time the synoptic wavelength pattern favors a southeast ridge over the eastern United States with a weak trough over the extreme northern Great Lakes moving east. This remains nearly 10-days out, but the general pattern can already be highlighted. This storm bares monitoring a bit more closely for those along the East Coast as several ECMWF runs have noted possible landfalls for the east coast from Fiona. It is all a matter of timing and whether the storm can pick up the weakeness in the trough to be carried northward. But what the ECMWF has been noting along with the ECMWF ensembles is that Fiona tracks far enough south to avoid the trough posing a threat to the Southeast and then tracking north and swinging up the coast.
Fig. 5- Current 28 August 2010 12utc 240hr ECMWF featuring South Carolina landfall of Fiona.
Right now historical odds actually do not favor a landfall based on the current position of 97L. But still those along the eastern seaboard will need to monitor this system closely. At this point I am not making any forecast as it remains well off in the distance, but it certainly has grabbed my attention if that means anything. None the less the three tropical system wills eventually make their way towards the North Atlantic shipping lanes. The tracks of these systems will likely cause a dramatic cooling of the anomalous warm Atlantic SSTs. Also these storms as the increase in altitude, may affect teleconnections and upstream blocking in this region. This may cause a few variables in the long term weather pattern; stay tuned. For now those along the East Coast can at the minimum expect increased surf, slight beach erosion, and a high threat of rip currents. I will have more on the tropical threat later this week. For now enjoy the 5-day heatwave and sunshine this week with dry weather.
Regional updating radar...
"Here north of Harrisburg 2010 statistics"
(Severe Weather Stats...)
Severe Thunderstorm Watches- 11
Severe Thunderstorm Warnings- 10
Tornado Watches- 1
Tornado Warnings- 1
Total Thunderstorms- 17
Flood Watches- 3
Flood Warnings- 2
Monthly Precipitation- 0.00inches
Yearly Precipitation- 26.54inches
Heat Advisories- 5
Excessive Heat Watches- 1
Excessive Heat Warnings- 1
90degree days- 37
Highest Temperature 101F (x2)