Area Forecast Discussion

WHAT HAS CHANGED

The forecast remains on track with temperatures cooling rather quickly early this morning given Canadian high pressure in place. The biggest uncertainty in the period is late this weekend into early next week. A number of complex features will hopefully become better resolved in the next couple of days.

KEY MESSAGES

- 1) Remaining cool today, with shower and thunderstorm chances returning late in the week and into the weekend.

DISCUSSION

KEY MESSAGE 1...Remaining cool today, with shower and thunderstorm chances returning late in the week and into the weekend.

The influence of Canadian high pressure is well underway with a marked fall in temperatures and dew points over the region. Relative to 24 hours ago, most locations have seen such readings fall by around 10 to 15 degrees. This translates to early morning temperatures that will start off in the upper 40s to 50s, locally into the mid 40s along the Allegheny Front. With low-level moisture being trapped within nocturnal radiation inversions, some patchy fog may materialize before daybreak. This would occur around the usual vulnerable locations like river valleys and other low-lying areas.

With a broad 1026-mb surface anticyclone anchored over the northeastern U.S., expect conditions to be fairly similar to yesterday/Monday. The one major difference will be the marked reduction in winds given weak gradients within the influence of this dome of high pressure. Today's high temperatures are forecast to reach the mid 70s to low 80s (upper 60s to low 70s in the mountains). This comes with continued low humidity with dew points mainly in the upper 40s to low 50s. Aside from some passing high cirrus, abundant sunshine is expected throughout the day. Light southeasterly gradients will favor an uptick in overnight temperatures. While still cooler than recent days, most can anticipate lows in the mid 50s to low 60s (mildest along the I-95 urban areas as well as near the Potomac River and Chesapeake Bay shorelines).

Behind an exiting longwave trough that pushes into the Canadian Maritimes, mid/upper heights begin to build in earnest. An expansive 594-dm mid-level ridge initially over the southern U.S. will flex northward in time. These above average heights remain a fixture in the forecast through Thursday. This supports a return of warmer temperatures along with building humidity levels. Daily highs push back into the mid/upper 80s, locally reaching the low 90s in the warm spots. The return of the humidity also favors milder nights with widespread lows in the 60s. Any shower and thunderstorm chances should hold off until Thursday given the stout ridge aloft quelling most precipitation chances. Most models keep such a threat mainly west of the Shenandoah Valley, especially closer to the Alleghenies.

A high latitude trough digging across southeastern Canada should be enough to nudge the upper ridge back toward the south-central U.S. This allows a cold front to push through early Friday which again brings a gradual cool down with decreasing humidity into Saturday. However, this reprieve is somewhat brief as additional disturbances in the atmosphere track toward the area late in the weekend. The overall jet pattern turns a bit more complex as an array of features enter the picture. A broad upper low retrogrades across the Gulf of America. At the same time, a myriad of shortwaves will eject across the central U.S. toward the Mississippi Valley. How these impulses behave along with where surface features reside will dictate any local weather impacts. However, all signs point toward a more active pattern on Sunday into early next week.

AVIATION /06Z TUESDAY THROUGH SATURDAY/

Canadian high pressure will support VFR conditions through mid- week. Weak gradients keep lighter winds than yesterday/Monday, with winds gradually shifting from east to southeasterly today. This ridge eventually edges toward the Atlantic coastline which will support mainly southerly winds on Wednesday into Thursday. Some storm chances emerge by Thursday, but these are currently expected to stay west of the area terminals.

A cold front moves through early Friday which results in some uptick in shower chances. This may come with a few restrictions if convection becomes more pronounced. In the wake of this frontal passage, winds turn more northerly with afternoon gusts to around 15 knots or so. High pressure building to the north results in a east to southeasterly wind for Saturday. This comes with likely VFR conditions.

MARINE

While most land observations have seen winds turn light, the cooler air overriding the warm waters have maintained a breezy northeasterly wind early this morning. Small Craft Advisories continue over the southern waters through 7 AM with gusts to around 20 to 25 knots. Across the middle Chesapeake Bay, gusts occasionally approach 20 knots, but these are more sporadic and infrequent in nature. However, gradients weaken through the day with gusts around 5 to 10 knots.

Thereafter, high pressure slowly pushes away toward the coast which results in a south to southeasterly wind for Wednesday and Thursday. Some southerly channeling effects may necessitate Small Craft Advisories for parts of Thursday, especially across southern sections of the Chesapeake Bay. A cold front tracks across the waters on Friday morning which favors a shift to north to northeasterly winds. These could near advisory criteria, but confidence is low. High pressure passing by to the north will result in a shift to east-southeasterlies for Saturday. Gusts up to 15 to perhaps 20 knots are possible.

TIDES/COASTAL FLOODING

Current tidal anomalies remain around zero feet. However, expect a gradual rise in water levels later today as high pressure pulls away from the Eastern Seaboard. This again pushes a number of sites into Action stage during the higher of the two astronomical high tides, particularly tonight and again late Wednesday.

LWX WATCHES / WARNINGS / ADVISORIES

DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory until 7 AM EDT this morning for ANZ534- 537-543.