Cutoff low progresses northeastward...
As another month ticks by on the intangible figment of time, another summer ceases to an close. By this midpoint in the year, many New Year's Resolutions have been broken, and summer vacation for students and teaches resume another year. Across the northern portions of North America, the seasonal transition is already evident. While no the leaves are not red and the snow has not fallen, but suddle changes are evident. The days grow shorter with the darkness of night already noticeable by 7pm and patchy ground fog resumes the typical fall like pattern during the early morning hours. Cold fronts now blow across the region evident in not only wind shifts, but temperatures. Instead of only a relaxation of humidity after a frontal passing, finally cooler temperatures are trickling in. We have passed the dog days of summer. And we have passed those 'wall of humidity' days. But now we enter a new chapter. While one could say every year this transition occurs, but unlike many things... the weather is never replicated. Increasing changes bring turmoil in the meteorology community as challenges impose. Skills in one set of forecasting weaken, while others strengthen. Autumn, while typically boring weatherwise, offers a renewed interest in winter forecasting. Rarely during the Summer months is a collaboration of model guidance such as the CMC, JMA, ECMWF, GFS, NAM, WRF, UKMET used in one setting. Fall allows for a resurgence of an active northerly jet that sinks southward into the lower 48. One could say forecasting thunderstorms is difficult as exact placement of rain vs. no rain is impossible at this point in time, but winter brings challenges like no other. Low pressure systems develop across the entire region affecting millions of people in the United States from flooding rains to blowing snow to slippery ice. So as the meteorological Summer closes August 31... another chapter in the endless meteorological notebook begins.
Like most any summer, the typical local news report of flash flooding occuring somewhere in the United States is common. Turn on the six o'clock news and a headline of flash flooding in Iowa or Arkansas or New York State or wherever... is expected. Flash flooding is second to heat in the number of weather-related deaths in the United States. This is probable due to most areas being in precipitation favored zones outside, say Death Valley. But a probable cause in the increasing flash flooding is not weather related at all. In fact many of these flash flood events likely could have been prevented in certain situations. The volume of water world wide remains a general steady constant. But any large rate of precipitation in a short period of time is a catalyst for flash flooding. With increasing urban sprawl, asphalt and concrete are quickly tearing away at Earth's natural filter... soil. As precipitation falls on asphalt, there is essentially no place to go, other than evaporation. It becomes runoff causing the ponding of water. Urban flooding is one of the leading causes of flooding deaths due to the ignorance of the 'turn around don't drown' philosophy. Also the disruption of streams, rivers, and topography displaces the natural flow of runoff precipitation in the soil. Recent years have acknowledged this problem with the advent of retention ponds, but the problem still remains a major concern. Take a typical thunderstorm... 30 minutes with about .75in of rain or so. 0.75in of rain over an area the size of a county covers a great deal of land primarily being soil (outside the major cities). But with urbanized areas this three quarters of an inch has no where to go other than drainage systems which often are clogged during Spring and Fall with debris. So a simple 0.75 reaks unnecessary havoc and minor flooding with ponding along roadways. This raises the question... Are flash floods really on the increase? Again that is one of those questions such as, "If a tree falls in a forest and no one is around to hear it, does it make a sound?" Yes flash flooding reports are increasing, but the volume of rain essentially is remaining a constant. Therefore the atmosphere is not necessarily responsible for the increasing reports, but more so they are increasing because of the growing population and growing human influence on the environment. Sure it is easy to blame mother Earth for a problem, but blaiming ourselves... well that is another story.
The week ahead looks rather benign weatherwise as soon as the interesting cutoff low in New England progresses northeast slowly. The cold front had already reached the I-95 corridor as of 8pm Sunday evening and the low pressure is churning up the East Coast.
Dry air aloft associated with a weak upper level circulation in southern Quebec is penetrating the troposphere in the Northern Middle Atlantic acting as an area of subsidence preventing additional rainfall for most areas Sunday night. But a weak banana high over Newfoundland will keep the low pressure in the vicinity through Monday night. As the winds back around the cutoff low, increasing PWATs will once again occur Monday. +1SD precipitate waters near 1.7in are progged on the H7 GFS moisture fields allowing the mention of precipitation in the forecast through Monday night. As the onshore easterly flow increases by Monday morning, low stratus will advect 1000ft or less ceilings into areas downwind of the Appalachians with an increasing threat of drizzle and light rain showers. HIRES NMM QPF levels add an additional 0.1-0.25in of rain regionwide on Monday with the higher amounts located over northeastern Pennsylvania. GFS QPF values remain similar also. A dreary forecast looks in store for most areas through Tuesday afternoon especially east of the Hagerstown-State College-Lock Haven line with drizzle and light rain along with patchy fog and low clouds. IFR conditions will be prevalent especially towards the coastal plain in Delaware, southeastern Pennsylvania, and Maryland. Marine influences will remain unfavorable for recreational use as the onshore flow will add the potential for a bit of oversplashing in favorable coastal flood locations along the Chesapeake Bay. 18utc 8/22/10 GFS also adds an interesting scenario on Wednesday with a weak coastal low forming along the quasi-stationary boundary in Virginia. This low then traverses northward with heavy rain into southern New England.
At this point my initial thoughts are that this could be a convective feedback problem, but the last few NAM runs have hinted at this scenario with a very minimal and weak 850hPa low in this region in the same time frame. In any case an unsettled approach to the forecast is the smart way to go for all areas through Wednesday evening. Temperatures are also below normal for a change courtesy of the stratiform rain. 24hr temperature changes from yesterday to today feature a widespread -10F departure for most areas.
By Wednesday night an anti-cyclinic flow resumes over the entire Northeast featuring a relative period of beautiful weather. Lower 80s and sunshine will be common Thursday through Saturday along with dewpoints in the 60s maintaining lower humidity. Looking at the long term patter, GFS and ECMWF collaborations are both initiating once again a strong ridging pattern with anomalous H85 thermals over the East Coast in the two week time frame from this current date.
At this point in time model guidance is likely struggling with the seasonal transition and probably is overestimating the amount of warmth. Longer wavelengths globally do not support ECMWF guidance in this period. While above temperatures are possible, these progged thermals would initiate record warmth in the mid 90s or so. While 90s are possible in any September, they are not the common place they once were in July and August. My thoughts in this time period feature above normal temperatures towards early to mid September, but nothing abnormal or above 92F or so. At this point the GFS maintains a semi-active jet flow in the Middle Atlantic, so precipitation anomalies should remain near normal in this time period also.
Monday- Drizzle and light rain regionwide with low stratus and cooler temperatures in the mid to upper 70s. Additional rainfall for most areas near .25in.
Tuesday- Mostly cloudy to cloudy with light rain, especially for areas in the east. Additional rainfall less than one tenth of an inch. Highs remain below normal in the mid to upper 70s. A few areas in western Pennsylvania may tap into some sunshine with highs in the 80s.
Wednesday- Improving conditions with clouds finally dissipating for eastern areas with sunshine by evening. Little to no additional rainfall. Temperatures near normal in the low to mid 80s.
Thursday- High pressures dominates the weather pattern with brilliant sunshine and a cool/dry northwest flow. High temperatures in the low 80s.
Friday- Remains a repeat of Thursday with high pressure in control and brilliant sunshine regionwide. Highs near to slightly below normal.
Saturday/Sunday- Sunshine dominates the weather pattern with a bit warmer temperatures in the mid to upper 80s, especially on Sunday.
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- 4.24inches
Yearly Precipitation- 26.54inches
Heat Advisories- 5
Excessive Heat Watches- 1
Excessive Heat Warnings- 1
90degree days- 31
Highest Temperature 101F (x2)