Marine Weather and Tide Forecast for Portsmouth, VA

KEY MESSAGES
1) Seasonable to slightly above average temperatures and dry conditions through Saturday.

2) Moisture and rain chances return Saturday night into early Monday.

DISCUSSION
As of 310 PM EDT Thursday...

KEY MESSAGE 1...Seasonable to slightly above average temperatures and dry conditions through Saturday.

The latest wx analysis continues to show a split flow pattern with a northern stream trough across the Northeast and a southern stream evident in the cloud pattern to the N and NE of Hurricane Isaias, with a trough in the vicinity of TX. Sunny skies are evident over most of the area, with scattered CU over the piedmont and high cirrus clouds moving into NE NC from the southern stream. A gradual increase in high level clouds will begin tonight, as upper level moisture spreads over the region well in advance of Isaias.
Temperatures are warm today with observations in the lower 80s, which could still rise another degree or so today before reaching the high. Today looks to be the warmest day of the week with temps trending down towards seasonal averages with highs on Friday in the upper 70s and in the lower to mid 70s on Saturday. Temps will likely be a few degrees cooler along the coast both days. Not necessarily impactful, but statistical guidance is on the order of 4-8F cooler than NBM guidance for lows late Friday night/early Saturday morning (lower 50s to lower 60s NBM vs. mid/upper 40s to upper 50s MOS). Dry conditions continue through Saturday.

KEY MESSAGE 2...Moisture and rain chances return Saturday night into early Monday.

By Sunday and Monday, there is increasing confidence that moisture gets pulled northward and then pushed eastward across the region as Isaias makes landfall and merges with an upper trough in vicinity of the Mid-South. Confidence in timing has increased some as the model guidance has came into better agreement for the onset of accumulated rainfall, showing the 06-12z Sunday timeframe. The 12z ensembles continue to show PW values aob 200% of normal along with a plume of instability surging N ahead of the trough. This could lead to heavy rainfall for areas and the chance for thunderstorms on Sunday. There is still spread between the 10th and 90th percentile QPF in the 12z/08 ensemble guidance, but amounts generally trended up. In general, the 10th percentile (reasonable low-end) accumulated precip through 00z Tuesday is 0.25-0.75", while the 90th percentile is on the order of 2.0-4.0". The 50th percentile (most likely case) shows a generalized 1.0-2.0" of rainfall, with the highest rainfall totals in the southern half of the area as there is a hint in the guidance of more enhanced rainfall along a coastal trough. The presence of a coastal trough would also suggest a band of drier air setting up somewhere over the area. This could lead to a large spread in rainfall totals across the area. The ECMWF, ECMWF ens, GFS, AI GFS, and the CMC all hint at portions of inland areas only receiving ~1.0- 1.5" with totals near the coast amounting to ~2.0-4.0". The coastal trough would also suggest an in situ wedge resulting in cooler temperatures and more stable conditions Sunday across the Piedmont.
Deeper moisture erodes Sunday night into Monday. However, there are lingering PoPs as the upper trough swings across the region. Behind this system, drier air will filter back in later Monday through the middle of the week. Temperatures should remain near average late this weekend through the middle of next week.

AVIATION /00Z FRIDAY THROUGH TUESDAY/
As of 657 PM EDT Thursday...

VFR conditions prevail for the 00z/09 TAF period as high pressure is centered offshore. Winds are mostly light and variable and will remain that way overnight. A dry cold front will cross through the area later tonight, which will cause winds to shift out of the N Friday morning. High clouds have begun building into the area from the south and will continue to move north through the night and into tomorrow morning.

Outlook: VFR conditions continue Saturday with increasing high clouds. The next chance for sub-VFR conditions arrives late Saturday into Sunday due to rain associated with the remnants of Hurricane Isaias. Flight restrictions could linger into early Monday, followed by gradually improving conditions.

MARINE
As of 355 PM EDT Thursday...

Key Messages:

- Sub-SCA conditions prevail through most of Saturday.

- Small Craft Advisories are likely Saturday night into early next week as the remnant of Hurricane Isaias tracks through the TN Valley and towards the local area, with strong high pressure well off the east coast.

The dry cold front is pushing S through the region this aftn, shifting winds to the N-NW at up to 10 kt. Expect northerly winds increase to 10-15 kt for a few hrs early Friday morning, but cold air is lacking with this surge, so no SCAs will be needed. Lighter winds become E-NE Friday afternoon with high pressure again settling over the area. Seas are expected to stay in 2-3 ft range through most of Saturday. Waves in the bay will be 1-2 ft.

Hurricane Isaias continues to push N, and is is forecast to make landfall in the western FL Panhandle early Saturday morning. While direct impacts are not anticipated locally, the interaction of the remnants of Isaias with an upper-level system is likely to lead to increased E-SE flow from Saturday night through Monday morning, with the peak in winds (20-25 kt w/ gusts to ~30 kt) occuring Sunday afternoon as the winds shift slowly to the S-SE. Seas will also substantially increase to 5-8 ft (up to ~10 ft well offshore), with 3-5 ft waves in the Chesapeake Bay (highest lower Bay) Regardless, Small Craft Advisories are likely during this time. A few gusts to gale force are possible Sunday morning across the lower Bay and southern coastal areas, shifting N through the aftn, but Gale Warnings are unlikely. The wind direction shifts to the W-NW Monday in the later half of Monday, while also gradually decreasing. A strong cold front could then pass through the area Tuesday into Wednesday, with elevated NW winds possible but timing is highly uncertain at this range.

AKQ WATCHES/WARNINGS/ADVISORIES
MD...None.
NC...None.
VA...None.
MARINE...None.