Marine Weather and Tide Forecast for Vienna, MD

KEY MESSAGES
1) Widespread showers and thunderstorms are expected this afternoon and evening with better potential for severe weather. The main threat is damaging wind gusts with large hail and a couple of tornadoes also possible.

2) Isolated showers and storms are possible on Wednesday, with mainly dry weather to end the week. Temperatures drop to slightly below average late this week.

DISCUSSION
As of 245 AM EDT Tuesday...

KEY MESSAGE 1...Widespread showers and thunderstorms are expected this afternoon and evening with better potential for severe weather.
The main threat is damaging wind gusts with large hail and a couple of tornadoes also possible.

An anomalously strong upper trough and sfc low are progged to deepen over the Northeast CONUS today while we remain under NW flow aloft.
The deepening has already started, with decent height falls being observed from the Great Lakes to Northeast over the past few hours.
At the surface, low pressure is developing along a cold front over the eastern Great Lakes. Convection has developed over western and central Ohio and is organizing into an MCS. Will need to watch this convection later this morning, but it appears that it will pass by just far enough to our W/SW as to not inhibit sfc heating (and thus instability) in a meaningful way for at least the eastern 2/3 of the area. There is a low chc of late AM/midday showers in the Piedmont from this feature. The surface low will continue to deepen over the Northeast today, and in fact, could deepen to 995mb with -2 to -3 sigma height anomalies over the northern Mid-Atlantic/Northeast. The synoptic features are impressive especially considering it is late July. There will be a belt of 35-45 kt 500mb winds on the SW side of the trough that is progged move over the local area during the afternoon. The above mentioned cold front will move toward the area later today and will cross the area tonight. Since the upstream MCS activity should pass far enough to our W/SW, temperatures will be able to climb into the upper 80s in most areas by the afternoon with lower-mid 70s dew pts. This, coupled with solid 700-500mb lapse rates around 7.0 degrees C, should allow MLCAPE to rise to 2000-2500 J/kg across much of the area.

While there are slight differences in the CAMs with respect to exact timing, convective mode, and convective evolution...the general consensus is for numerous tstms to initially develop just to our north during the early-mid aftn before moving through the area from NW-SE during the late aftn-evening. The most likely timing for storms is from 3-7 PM N/NW, 5-8 PM from south central VA to Richmond to the MD eastern Shore, and from 6-11 PM in SE VA/NE NC. Whether the storms consolidate into a line or remain more discrete is not known. The main threat appears to be damaging wind gusts, which would be fairly robust whether storms remain more discrete or consolidate into a line/line segments. There is also a large hail threat, mainly across the NW 2/3 of the area where storms are more likely to be discrete. Lastly, a couple of brief tornadoes cannot be ruled out, especially closer to the coast where the winds will be more backed. SPC has maintained a Slight Risk (level 2/5) for the entire area, with 30% wind probs and a 5% tornado risk for much of the area. The main bust potential will be that the developing MCS over OH becomes stronger and tracks farther NE than expected. If this happens, it could potentially contaminate our environment and lead to a lower than anticipated severe threat later today. Storms will be moving quickly, which will act to limit flooding, but cannot rule out localized flash flooding, mainly in urban/flood prone areas.

KEY MESSAGE 2...Isolated showers and storms are possible on Wednesday, with mainly dry weather to end the week. Temperatures drop to slightly below average late this week.

There is some uncertainty in regards to rain chances in the wake of the cold front on Wednesday. With the closer proximity of the upper low, there cold be some showers on Wednesday, but guidance has trended down on this. Have maintained slight chance PoPs across the northern portion of the forecast area for consistency. Thursday through Friday looks mostly dry, with near to slightly below average temperatures, as LLVL flow will continue to be out of the NW. Tstm chances may return by late this weekend.

AVIATION /12Z TUESDAY THROUGH SATURDAY/
As of 640 AM EDT Tuesday...

Patchy IFR-LIFR stratus/fog that is impacting the terminals now should burn off by 12-13z. Otherwise, VFR conditions will prevail through mid aftn. The main aviation concern through the period is the potential for numerous tstms during the 20-03z timeframe. While the exact timing of these storms remains somewhat uncertain, still think that most of the terminals will be impacted by these storms. Convection initially develops just to the north of the CWA during the early-mid aftn before tracking from NW-SE through the area from late aftn-late evening. A cold front moves through early Wed AM, with a period of MVFR-IFR CIGs possible later tonight after the convection exits. Will go ahead and add TEMPO groups for TSRA with the 12z TAFs. Some of the storms will likely produce strong to severe winds.

Outlook: Isolated showers/tstms are possible Wednesday, but PoPs are much lower than on Tuesday. Drier conditions return by late week.

MARINE
As of 310 AM EDT Tuesday...

Key Messages:

- Small Craft Advisories have been issued for the Chesapeake Bay, Lower James River, and northern coastal waters today.

- Scattered to widespread storms are expected this afternoon into this evening with the potential for strong winds, hail, waterspouts, and frequent lightning.

The latest surface analysis depicted an anomalously strong upper level trough across the East Coast. At the surface, high pressure remains offshore with a lee trough developing inland ahead of a developing surface low in the Ohio Valley and associated cold front. Models disagree with the placement of the aforementioned surface low today with some showing it over PA and others over VA and the local waters. Meanwhile, WPC shows the low across northern PA/NY. In any case, the low will deepen today before pushing a cold front across the local waters tonight. Ahead of the front, S winds increase to 15-20 kt with gusts up to 25 kt this afternoon across the local waters. As such, SCAs have been issued for the Chesapeake Bay, Lower James River, and northern coastal waters this afternoon into this evening. SCAs begin a bit earlier (10 AM EDT) for the upper bay to account for some uncertainty with respect to the timing of the stronger winds. However, the best chance for SCA conditions remains the late afternoon into the early evening for all locations under SCAs. Will note that winds may become SSE (or even SE at times) late this afternoon ahead of convection. SCAs end at 1 AM Wed. Additionally, scattered to widespread storms are expected this afternoon into this evening across the local waters with strong to severe storms possible. Strong winds, hail (potentially large/severe), waterspouts, and frequent lightning are all possible with the storms this afternoon into this evening. These storms will likely interfere with the synoptic flow. This could result in either weaker winds outside of convection due to cloud cover interfering with WAA or stronger winds overall due to the widespread convection. Storms taper off from W to E tonight as a cold front moves across the local waters.

Winds become W 10-15 kt tonight behind the cold front, become NW late tonight into Wed morning with a secondary surge. Several CAMs show the potential for SCAs conditions with the NW surge late tonight into early Wed. However, confidence is too low to extend the SCAs beyond early tonight at this time. Additionally, the lull in SCA criteria winds tonight appears long enough to keep any secondary surge SCAs separate. Winds diminish to 5-10 kt by Wed afternoon as high pressure begins to build into the area. Another brief surge of NW winds is possible Wed night into early Thu morning. Otherwise, a period of benign marine conditions is favored from late this week into early next week.

Waves and seas build to 2-3 ft and 3-4 ft respectively later this afternoon into this evening before subsiding Wed. A period of 4-5 ft seas is possible across the northern coastal waters late this afternoon into this evening.

AKQ WATCHES/WARNINGS/ADVISORIES
MD...None.
NC...None.
VA...None.
MARINE...Small Craft Advisory from 10 AM this morning to 1 AM EDT Wednesday for ANZ630.
Small Craft Advisory from 1 PM this afternoon to 1 AM EDT Wednesday for ANZ631-632-634-639-650-652-654.