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Weather Wars: 7 Day Forecast Battle
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Drier Air Moving in... Storms Return for 4th-5th

The same front that sparked our severe weather on Thursday has been very slow in getting out of the area. However, it looks like drier air will finally start filtering into the area tonight and into tomorrow (below).

NAM model for Sunday, showing humidity at about 20%!!!

As for the work week, it looks like the heat and humidity will return (like that's new) for Tuesday, and really get bad for the 4th of July. We will need to watch the chance of storms for the 4th, as moderatly unstable air will meet an approaching cold front. I don't believe that we will see a wide-scale severe weather event on the 4th due to an absense of sheer in the atmosphere--- but spotty storms may be possible.

The real interesting day this week appears that it will be Thursday, where there may a significant severe weather event...stay tuned.
By John Y. On Saturday, June 30, 2007 At 1:44 PM


The National Weather Service has issued a severe thunderstorm watch in effect for the entire D.C. metro area until 11:00 tonight...damaging winds will be the biggest threat.

A sqaul line appears to be developing in Central Pennsylvania (below) and may cross the area in the early evening. Ahead of this line, a couple of isolated strong storms may pop up through 6pm. Stay tuned for updates....
By John Y. On Thursday, June 28, 2007 At 2:38 PM



A highly unstable airmass combined with a slow moving cold front will begin to spawn some strong thunderstorms this afternoon and moreso tomorrow.

One small pop-up storm already caused some small hail over Eastern Fairfax county at around 1:30pm today--- and more additional storms are possible later this afternoon.

Additional updates will be posted if watches/warnings are issued.

Radar image at 1:34 showing the dying stages of the thunderstorm
over Eastern Fairfax county and additional storms building over the
Appalachians and Northern Maryland.
By John Y. On Wednesday, June 27, 2007 At 12:30 PM

Pleasant weather to dominate...

Temperatures for the remainder of the week will remain in the low to mid 80s, under mostly sunny skies with low humidity. While there is a slight chance for a storm Friday, for the most part, D.C. weather will be wonderful the next several days...

Satellite image taken Wednesday showing the last of the
clouds from the cold front moving out of the area, and drier more
confortable air moving in.

By John Y. On Wednesday, June 20, 2007 At 12:16 PM


11:00 UPDATE: Severe Thunderstorm watch has expired... there may be a couple of isolated strong storms later tonight, but the main threat has diminished with little rain or damge to speak of from earlier today.


Strong storms have developed across the area and have prompted the National Weather Service to issue a Severe Thunderstorm Watch for the entire D.C. metro area until 11:00 EST.

Discussion: A developing line of storms along I-81 will strengthen and slowly push north and east. While there may be a couple of storms ahead of this line, the main threat to the area will come later this evening around the 6-9 time frame..
By John Y. On Tuesday, June 19, 2007 At 3:46 PM

Severe Thunderstorms possible Tuesday

With temperatures expected to climb well into the 90s tomorrow, a round of strong storms is possible. The most likely areas to see severe storms are the northern suburbs of the D.C. area.

A strong cold front will approach the area and provide the lift for the storms, as indicated on the lifted idexes forecast map below. While there may be a couple of scattered storms in the afternoon time period, several computer models indicate that a squall line will develop and cross the area as shown on the NAM model forecast (right) in the late evening hours.

(Above) NAM model forecast for the late evening hours Tuesday showing a squal line extending from Eastern Canada to the northern suburbs of D.C.
Damaging winds are usually the most dangerous risk with squal lines.

(Above) Lifted indexes are projected to be in the -4 to -5 range in the afternoon and evening hours. Note: lifted indexes are a measure of the instability in the atmosphere...

>0: any thunderstorm development is unlikely
0 to -3: Severe thunderstorms possible but not likely
-3 to -6: Severe thunderstorms possible
-6 to -9: Severe thunderstorms likely
<-9: GET OUT!
By John Y. On Monday, June 18, 2007 At 8:44 PM

Heat to return...

With an onshore breeze, temperatures hit
just 69 degrees on Wednesday and 70 yesterday. However, winds will shift to the south and west as we head into the weekend pushing temperatures into the 80s and near 90 by Sunday.

The heat will continue to surge into early next week, with a number of weak systems skirting to our north. While there may be a brief relief from the heat with the passage of a weak cold front on Wednesday-Thursday, long-term models continue to show dangerous heat and humidity for the remainder of the month.

The 18z NAM model for Sunday--- showing temperatures in the area hitting 90 degrees and more heat building west (above).

GFS model showing temperatures getting close to the 100 degree mark on the 26th and 27th of June (above and right).
By John Y. On Friday, June 15, 2007 At 10:30 PM

Warming Up for Father's Day

Tonight and Tomorrow:
Temperatures tonight should be seasonably pleasant, dipping into the mid 50s around DC, to near 50 closer to the Maryland state line. Showers will become more of a threat later tomorrow afternoon as a another upper level disturbance slides southwestward from Newfoundland. High temps tomorrow should be able to sneak into the lower to mid 70s under mostly cloudy skies.

Temperatures by Father's Day will likely soar into the 90s as an area of high pressure slides to our south, transporting warm and humid air into the region. After Sunday, however, a general cooling trend is expected, and temperatures should return to near normal levels by early next week. A cold front is expected to cross the Mid Atlantic Wednesday afternoon, and will further lower temperatures into the lower 80s.

Image: Several members of the Short Range Ensemble Forecast Model (SREF) predict high temperatures near 90-92 degrees on Sunday afternoon. The average (bottom right) is near 88-90 for the immediate metro region.

Image courtesy of the Penn State E-Wall

107th US Open Weather=Fantastic:
While the weather for the first round of the 107th US Open at Oakmont Country Club wasn't quite ideal, mother nature should cooperate nicely for the remaining three rounds of the Open Championship. Temperatures will hover just around 80 degrees Friday, Saturday, and Sunday, and skies should remain mostly sunny to party cloudy. Very isolated showers may develop just east of the Pittsburgh, PA area, but should not be a threat to play.
By Lee Carlaw On Thursday, June 14, 2007 At 7:57 PM

Cooler Weather The Rest of the Workweek

A few showers and thunderstorms are presently traversing areas south and west of the immediate DC metro area. An intense, severe thunderstorm currently over Herndon and Fairfax in Virginia produced nickel to quarter size hail in Reston, and will affect Manassas and Manassas Park within the next 30 minutes to an hour.

Regional forecast models do indicate the potential for the main line of severe storms (currently located in Central Pennsylvania) to propogate southward into central Maryland between 6 and 8 PM tonight. If this does ocurr, and the line maintains most of its intensity as forecast, areas from DC northward would likely experience a second wave of showers and thunderstorms before things die down with the loss of daytime heating.

Tomorrow through Friday:
Temperatures tomorrow should only rise into the lower 70s partly to mostly cloudy skies. Any precipitation appears unlikely at this point.

Low temperatures Thursday and Friday nights will range from 56 in northern areas of Maryland to 63 in places like Fredericksburg and central Virginia. Obviously, the air conditioner can get a brief rest before near to slightly above normal temperatures slide back into the region for the weekend.

Saturday and Sunday:
Under sunny skies, temperatures on Saturday and Sunday will rise into the mid and upper 80s across the Mid Atlantic, and we may even break 90 on Sunday as winds swing around to the southwest, funneling warm air into the area.

Large Earthquake Strikes off Guatemala:
According to CNN and the USGS, a large, 6.8 magnitude earthquake struck just off the coast of Guatemala at 3:30 PM Eastern Daylight Time. Due to the quake's depth, distance from land, and overall magnitude, damage will likely remain relatively minimal and aftershocks and tsunami are not a threat.

Image courtesy of the USGS Hazards Program of Northern California
By Lee Carlaw On Wednesday, June 13, 2007 At 3:06 PM

Yet Another Severe Thunderstorm Event


More isolated severe storms will likely develope over the next several hours.

Severe Thunderstorm Watch in effect until 9pm EST for the entire D.C. metro area with the exception of extreme southern Virginia south of Fredericksburg.

Discussion: Same setup as yesterday. Unseasonably cold mid-level lapse rates will again make damaging hail the main risk. Expect the main cluster of storms to move over the area in the 5-7 pm time frame.

Satelite imagery showing the storms developing over nothern Pennsylvania with the center of low pressure off the New Jersey coast

By Lee Carlaw On At 2:25 PM

Severe Thunderstorm Watch in Effect until 8PM EST

8:00 Update

Severe Thunderstorm Warninds in effect for: Culpeper, Rappahannock, Stafford, the city of Fredericksburg, and Spotsylvania Counties in Virginia

Discusion: All of the severe activity has cleared the D.C. metro area...

Clinton, MD reported 2" size hail at 6:17 PM!!!


Severe Thunderstorm Warnings in effect for: Montgomery, Talbot, Queen Anne's, and Western Dorchester Counties until 6PM

Storms are approaching the DC metropolitan region, moving in a pecuiliar fashion to the south and southwest (usually, storms in the Mid Atlantic travel west to east) at about 20-30 miles per hour. Expect thunderstorms with heavy downpours, wind gusts 40-60 mph and cloud to ground lightning to move inside the beltway within the hour.

Severe Thunderstorms are firing east of the Chesapeake Bay over Talbot and Queen Anne's counties and should move into southern St Mary's county in two hours or so.

Above Image: Radar at 5PM shows thunderstorms moving southwest towards the District. Some of the more intense storms contain small hail. (GRLevel3)

Previous Post
The air mass over us has become moderately unstable this afternoon with temperatures well into the 80s. Additionally, unseasonably low freezing points will make damaging hail the main threat with these storms.

Expect the main complex of storms to move over the area around the 6-7 pm hour.
By John Y. On Tuesday, June 12, 2007 At 1:55 PM

Stubborn Closed Low to keep us Unsettled

**Note: A new member--John Yarchoan--has joined DCweather.com and will add posts from time to time. We are also planning on adding a "weather wars" section in which we both battle over temperatures and weather forecasts**

An offshore closed low seen on visable satellite
(right) will keep the D.C. metro area unsettled for at least the first part of the work week. While we are not talking about a washout, scattered showers and thunderstorms are possible on Monday and even more likely on Tuesday and Wednesday. On a positive note, due to a lack in upper level winds, any shower or storm that may pop up in the afternoon hours has the potential of dropping a solid quarter to half an inch of rain, and thus may help us make a dent into the rainfall deficit. Also, temperatures will strugle to get out of the 70s for much of the week.

The closed low seen off the coast of Cape Cod (above)

The problem can be more clearly identified if we take a look at the 500 mb chart (left). A small disturbance which is what is left from the severe thunderstorm outbreak across the east on Friday is being forced between the troph from the closed low and a developing ridge across the deep south. When this happens, unsettled weather often takes control. The pattern will stay in place at least until Wednesday because as the disturbance drifts south, it will drag the closed low with it.

This can best be seen on the 500 mb chart for Wednesday. In this frame, the disturbance as strengthened and drifted south to a position around Mobile, AL. As it has done so, it has forced the closed low south and west, thus continuing to give us unsettled weather.

500 mb chart for Monday (top) and Wednesday
(bottom) showing the closed low across the
Northeast and a weak disturbance drift from
the Ohio River Valley to the deep south .

Images courtesy of Unisys Weather and NOAA

By John Y. On Sunday, June 10, 2007 At 4:27 PM
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