Marine Weather and Tide Forecast for Tilghman Island, MD

KEY MESSAGES
- 1) Remnants of Hurricane Isaias to bring rain, breezy to windy conditions, and the threat of tornadoes Sunday.

- 2) Above normal temperatures Tuesday will fall the second half of the week as moisture starved fronts move through.

DISCUSSION

KEY MESSAGE 1...Remnants of Hurricane Isaias to bring rain, breezy to windy conditions, and the threat of tornadoes Sunday.

Hurricane Isaias is forecast to move north toward the TN Valley today as it weakens and eventually loses all of its tropical characteristics. The mid-level circulation strengthens and becomes a large cutoff low later tonight. To the east, surface high pressure over the Northeast today moves offshore off southern New England tonight.

The interaction of Isaias' remnants to the west and surface high to the east will create a large area of convergence / broad frontal zone over the Mid-Atlantic. There is good model agreement that rain steadily moves from south to north this evening into tonight as forcing for ascent increases with some mid-level height falls in response to the approaching cutoff low. It'll take a bit of time for the surface to truly saturate, so any rain late afternoon to this evening in Central Virginia is going to be light. Eventually, tropical moisture with PWATs at or above 2" surges in after midnight.

It is worth mentioning again that conditions are favorable for a widespread soaking rain, and 00Z models have responded with more QPF across the region. Forcing for ascent increases tonight, and especially Sunday morning into Sunday afternoon as mid-level heights fall with the approach of the cutoff low. Enhanced low-level convergence and moisture transport will peak Sunday morning into early Sunday afternoon. Strengthening easterly flow into the Blue Ridge could also produce enhanced rain amounts due to orographic effects. As a result, forecast rain amounts have increased to 1.5- 2.5", with locally higher amounts of 3-4" possible. No Flood Watch issued with this forecast update as the heaviest rain is forecast to occur in areas that can handle it, but will continue to monitor the latest guidance today.

Another noticeable trend in the 00Z guidance is the higher probabilities for severe weather Sunday afternoon. The main synoptically forced rain moves north of the area early Sunday afternoon. Rapid height-falls and increasing low-level winds provide a few hour window where some breaks in the clouds area possible, leading to pockets of instability. A surface wave of low pressure moves right through our area Sunday afternoon. Forecast soundings indicate large, swooping hodographs, with ample shear in place to support organized severe convection. Clusters of thunderstorms develop along the leading edge of the surface low pressure, with tornadoes and damaging wind gusts as the main threats (looks mostly to be east of the Blue Ridge). The NAM12, RRFS, and 06Z HRRR all show this scenario. The big question still remains how much instability actually develops. SPC has introduced a Slight Risk for Sunday just to our south, with a Marginal Risk north to the MD/PA border.

The strong mid-level circulation is going to bring windy conditions to the Alleghenies and Blue Ridge, starting late tonight into Sunday morning. Wind Advisories have been issued for downsloping easterly winds in the Alleghenies (western Garrett, western Grant, and western Pendleton Counties), and the Blue Ridge. The current forecast for peak wind gusts is 40-50 mph in the Alleghenies/Blue Ridge and 20-30 mph elsewhere.

The remnants of Isaias push offshore Monday, bringing an end to any lingering showers. While conditions return humid and warm Monday, there likely won't be much in the way of shower activity due to subsidence behind the departing trough.

KEY MESSAGE 2...Above normal temperatures Tuesday will fall the second half of the week as moisture starved fronts move through.

Brief ridging in the wake of the tropical remnants may bring high temperatures Tuesday to near 80 degrees. Low pressure passing north of the area will send a cold front southward Tuesday night. Most of the rain likely remains across Pennsylvania, although some showers may sneak southward. Showers will also be possible along the Allegheny Front. There's still some significant model spread on the progression of this front during the middle of the week, with a large spread of temperatures possible on Wednesday. However, the general trend will be downward for the second half of the week. A wave of low pressure or cold front may approach toward the end of the week which could bring some rain, but model spread remains high on the details.

AVIATION /07Z SATURDAY THROUGH WEDNESDAY/
VFR conditions prevail through this evening, then conditions deteriorate tonight into Sunday as the remnants of Hurricane Isaias impact the region. IFR conditions are likely to overspread all terminals late tonight through Sunday afternoon, as periods of rain and gusty winds cross the region. There could be a few hour period of thunderstorms Sunday afternoon, some of which could be severe, though this threat is greatest east of the Blue Ridge. Conditions improve Monday morning, with the return of VFR conditions. Northwest winds gusting around 20-25 knots possible Monday afternoon.

VFR conditions are most likely Tuesday and Wednesday. A dry cold frontal passage Tuesday night may be followed by gusty northwest winds Wednesday.

MARINE
Marine conditions are expected to deteriorate tonight through Sunday night as the remnants of Hurricane Isaias move across the Mid- Atlantic. SCA conditions develop this evening in the central Chesapeake Bay, then across all the waters tonight. The strongest winds, gusts around 30 knots, will occur Sunday morning. Special Marine Warnings may be needed Sunday afternoon for thunderstorms capable of producing strong wind gusts and waterspouts. The remnants of Isaias move offshore Monday morning, but SCA conditions continue through Monday evening due to gusty northwest winds in its wake.

Sub-SCA conditions are likely Tuesday with brief high pressure. SCA conditions are possible in southerly flow Tuesday night and northwesterly flow on Wednesday as a cold front pushes through.

TIDES/COASTAL FLOODING
Widespread tidal flooding is likely during the Sunday afternoon to evening high tide cycle. Several conditions are coming together to produce a surge of water up the Chesapeake Bay and Potomac River.
Strong onshore winds coming during the highest of the daily tides, which happens to coincide with the New Moon are going to produce minor tidal flooding at most forecast locations. A Coastal Flood Watch has been issued for Annapolis where Moderate tidal flooding is possible. Water levels quickly decrease Monday morning as offshore winds follow.

LWX WATCHES/WARNINGS/ADVISORIES
DC...None.
MD...Coastal Flood Watch from Sunday afternoon through Sunday evening for MDZ014.
Wind Advisory from 8 PM this evening to 11 AM EDT Sunday for MDZ509.
VA...Wind Advisory from 2 AM to 11 AM EDT Sunday for VAZ507-508.
WV...Wind Advisory from 8 PM this evening to 11 AM EDT Sunday for WVZ501-505.
MARINE...Small Craft Advisory from 1 AM to 11 PM EDT Sunday for ANZ530>533-535-536-538>542.
Small Craft Advisory from 7 PM this evening to 11 PM EDT Sunday for ANZ534-537-543.