Marine Weather and Tide Forecast for Arnold, MD

KEY MESSAGES
- 1) Hotter conditions return Thursday with potential for severe thunderstorms and strong gradient winds.

- 2) A strong low pressure system could bring the threat for heavy rainfall and thunderstorms late Sunday through Monday.

- 3) An active upper-level pattern could bring continued rain chances through mid-week.

DISCUSSION
KEY MESSAGE 1...Hotter conditions return Thursday with potential for severe thunderstorms and strong gradient winds.

An anomalous low pressure system for June is forecast to track across the Great Lakes tonight and into central Quebec Thursday.
The associated cold front will push south through the forecast area Thursday afternoon into Thursday night. Attendant to this low will be a very strong wind field, with the strongest low level jet forecast to cross the area during the morning before retreating to the northeast during the afternoon.

Heat: A hotter airmass will quickly advect over the area Thursday.
Good mixing and a downslope component to the wind will result in temperatures pushing into the 90s for many locations (assuming clouds aren't too persistent). While it will be more humid than the past couple days, dew points should mix down during peak heating, resulting in heat index values in the mid 90s to perhaps near 100.

Wind: The strong wind field combined with good mixing of southwest to west winds will result in breezy conditions through the day.
There are two windows of potentially stronger winds. If mixing occurs quickly enough in the morning, some of the low level jet will be able to mix down. Then in the afternoon, pressure rises behind the front (or at least a more aligned with height westerly winds in the wake of the lee trough) combined with a tightening pressure gradient could result in some stronger winds, especially in northwestern parts of the area. A Wind Advisory has been issued for Garrett and western Grant Counties, which will be both closer to the stronger wind fields as well as see frontal passage in the afternoon. The advisory may need to be expanded to more high elevation zones based on additional data, though clouds and remnant showers may limit mixing of the LLJ during the morning. Elsewhere across the area, 25-40 mph gusts will be common throughout the day.

Severe thunderstorms: The thunderstorm threat for Thursday remains uncertain due to competing factors and upstream impacts. The evolution of thunderstorms across the Midwest today/tonight may have an impact on what develops across the local area on Thursday. Many models have some sort of remnant convection reaching the Allegheny Mountains around dawn, likely weakening significantly in the process. If the storms don't dissipate entirely, the remains or outflow could reignite along the surface trough to the east during peak heating. If convective perturbations are more absent, it's possible clouds could inhibit instability development. In addition, the downslope (drying/subsidence)
component to the winds could cap convection as well. However, more models are showing at least isolated to scattered development, especially ahead of the surface trough (roughly splitting the forecast area NE to SW) during the afternoon hours. While the LLJ will pivot away by this time, there will still be plenty of shear and enough instability for organized thunderstorms producing a damaging wind threat, especially given the DCAPE available. Localized considerable wind damage can't be ruled out. Most of the storms should exit the area by early to mid evening.

The cold front will push southward Thursday night. The front will absorb an area of low pressure to the south, likely originating from Arthur. There is still some spread with how far north additional rain reaches, but some rain chances will linger across the southern half of the area in particular through Friday morning. Skies should clear through the day with lowering humidity and seasonable temperatures.

KEY MESSAGE 2...A strong low pressure system could bring the threat for heavy rainfall and thunderstorms late Sunday through Monday.

After a quiet weekend, the upper-level pattern becomes quite amplified by late Sunday. However, there are a lot of moving pieces of upper-level energy to keep track of when it comes to determining how our area might be impacted. There is growing confidence in a fairly potent area of low pressure developing somewhere across the northern/central Plains on Sunday, before moving into the OH Valley/Great Lakes region into Monday. The big question is going to be where the storm actually tracks. Right now all hazards are at play with this system, ranging from severe thunderstorms to potentially flooding rainfall depending on the track. This system may be working with a connection to the Gulf of America, with 1.5 to 2+ sigma PWAT anomalies, so there will be some substantial moisture to work with. Our area has been in a pretty longstanding drought though, so in a best case scenario, perhaps this system bring mostly beneficial rainfall to some very dry areas. Again, this is a very complex upper-level pattern that is hard to nail down at this stage, but it is worth noting that this could be the next potentially hazardous weather system.

AVIATION /19Z WEDNESDAY THROUGH MONDAY/
VFR conditions should prevail into Thursday morning. A sprinkle could occur this afternoon, but no impacts are expected. Winds may vary between southwest and southeast through the night, gradually strengthening toward dawn. There will be a strong low level jet moving overhead at this time, so there could be some marginal LLWS depending on how much the surface layer decouples.
Only included this mention at CHO and MRB for now.

Southwest gradient winds quickly increase Thursday morning, and could gust in excess of 30 knots at times. There remains some uncertainty in convective coverage and timing for Thursday, which could depend on upstream convection today. Latest guidance suggest a mid afternoon peak in probabilities. PROB30 groups have been used due to the combination of low coverage and low confidence. MRB may largely miss out on the potential for stronger storms, but the strongest could produce severe gusts elsewhere. A second round of showers could affect portions of the area Thursday night into Friday morning, but impacts are uncertain. Skies should gradually clear Friday with gusty northwest winds.

Gusty NW winds around 20-25 kts can be expected Saturday, but will lighten up and turn out of the south into Sunday at around 12 to 18 kts. VFR conditions are expected throughout this time.

MARINE
Southwest to southeast winds are expected through tonight and will gradually increase, supporting Small Craft Advisories for portions of the waters by late this afternoon and most waters tonight. High end advisory conditions are likely Thursday as strong low pressure passes to the north. Issued a Gale Watch for the upper tidal Potomac River and portions of the northern Chesapeake as multiple models show 35 kt winds in the mixed layer. However, realization of these conditions (and thus the need to upgrade to a warning) will depend on: (1) mixing quickly during the morning hours while the low level jet is overhead, which might be thwarted by clouds and showers, and (2) the pressure gradient being strong enough when winds shift westerly in the afternoon for a second peak in gusts. By afternoon, the strongest gusts will be near the shore as air temperatures rise into the 90s. Special Marine Warnings may be needed during the afternoon and early evening as scattered strong to severe thunderstorms potentially track over the waters.

There may be a bit of a break in the winds Thursday night as the front slows down. Marginal Small Craft Advisories may be needed for portions of the waters Friday in northwest flow.

Winds on Saturday will be out of the NW and near SCA criteria in the wake of a secondary cold front. Winds will likely taper off into Saturday evening as they gradually turn out of the south.
We then get into a favorable southerly channeling pattern, which could necessitate SCAs once again Saturday night into Sunday.

TIDES/COASTAL FLOODING
As an increasing southwesterly flow evolves ahead of a cold front, anomalies will peak tonight into Thursday morning.
Several locations may experience minor tidal flooding late tonight into Thursday morning. Advisories have been issued for Annapolis and Havre de Grace, where confidence is currently highest. Annapolis may even approach moderate flood stage.

LWX WATCHES/WARNINGS/ADVISORIES
DC...None.
MD...Small Craft Advisory from midnight tonight to 9 AM EDT Thursday for MDZ008.
Gale Watch from Thursday morning through Thursday evening for MDZ008.
Coastal Flood Advisory from 5 AM to 1 PM EDT Thursday for MDZ014.
Coastal Flood Advisory from 10 AM to 4 PM EDT Thursday for MDZ508.
Wind Advisory from 7 AM to 6 PM EDT Thursday for MDZ509-510.
VA...None.
WV...Wind Advisory from 7 AM to 6 PM EDT Thursday for WVZ501.
MARINE...Small Craft Advisory from midnight tonight to 9 AM EDT Thursday for ANZ530-535.
Gale Watch from Thursday morning through Thursday evening for ANZ530-531-535-538.
Small Craft Advisory until 9 AM EDT Thursday for ANZ531-538.
Small Craft Advisory until 8 PM EDT Thursday for ANZ532>534- 539>543.
Small Craft Advisory from midnight tonight to 8 PM EDT Thursday for ANZ536-537.