textproduct: Caribou

WHAT HAS CHANGED

930am update...Issued Flood Watch for flash flooding this afternoon through 3am tonight for Central Highlands into the North Woods

- Let Dense Fog Advisory expire for coastal Washington County at 9am

- Increased qpf amounts to indicate basin average amounts around 1 inch in the Penobscot Basin

- 12z Aviation discussion update

KEY MESSAGES

1) Widespread showers and thunderstorms driven by a backdoor cold front and a humid airmass will bring the potential for localized heavy rainfall today.

2) Cold front approaches Saturday night and passes through Sunday into Monday, bringing widespread rain and thunderstorm chances.

DISCUSSION

KEY MESSAGE 1...Widespread showers and thunderstorms driven by a backdoor cold front and a humid airmass will bring the potential for localized heavy rainfall today.

KEY MESSAGE 1 DESCRIPTION... Today presents a challenging forecast as a quasi-stationary front over the Miramichi Highlands of New Brunswick slowly drifts southwestward as a backdoor cold front. Within the warm sector across much of the CWA, PWATs will surge to near 2 inches approaching daily record values at KCAR and representing 200-230 percent of normal. This anomalous moisture is accompanied by very deep warm cloud layer (12-14kft) and low LCLs, setting the stage for highly efficient, warm rain processes in any convection that develops today.

Despite the development of a light southeast wind, areas from Greater Bangor to the Central Highlands and the Baxter region will see peeks of sunshine today. This surface heating will allow diurnal instability to build, with global and hi-res CAM guidance indicating 700-1250 J/kg of SBCAPE with some up to 1500j/kg. The most favorable instability will pool southwest of the advancing backdoor front in northeastern Aroostook County, as well as ahead of a weak surface trough over New Hampshire and SE Quebec. This places much of the CWA, away from the stable marine influence of the seabreeze, in a favorable environment for convective initiation.

Heavy Rain / Flash Flood Threat: We will need to monitor the excessive rainfall threat starting early this morning. Several CAMs indicate the potential for training heavy showers in northeastern Aroostook County just ahead of the advancing backdoor front. As convection blossoms late morning into the afternoon across the broader warm sector, forecast soundings display classic "skinny CAPE" profiles, further corroborating an efficient precipitation and heavy rainfall threat.

While dry antecedent soil conditions will initially help absorb runoff, the overall environment is highly favorable for excessive rainfall. Slow storm motions, near-record PWATs, extreme precipitation efficiency, and storm vectors paralleling the boundary suggest a moderate to high probability of training convective elements. 1hr flash flood guidance (FFG) is 1.5-2 inches with 3hr FFG 2-3 inches. A few limiting factors to the flash flood threat do exist: some guidance depicts slightly faster storm motions, and poor mid-level lapse rates may suppress overall instability and convective coverage in some locations. However, much of the hi-res CAMs do generate large areas of convection so will need to monitor the trends. WPC has the entire area in a marginal risk of excessive rainfall which means isolated flash flooding possible.

Temperatures...A stark temperature gradient will setup across the region today. Behind the backdoor front, temperatures will fall quickly into the 60s (with a low probability of a few spots dropping into the 50s). Elsewhere in the warm sector, expect a muggy afternoon with highs in the upper 70s to low 80s and muggy dewpoints in the upper 60s to low 70s.

KEY MESSAGE 2...Cold front approaches Saturday night and passes through Sunday into Monday, bringing widespread rain and thunderstorm chances.

KEY MESSAGE 2 DESCRIPTION... Shortwave over the Great Lakes region moves towards Maine and enters New England by Sunday. Rain moves in from west to east, lasting Sunday into late Monday. Sunday seems to have decent instability - with low-level lapse rates around 7-8 C/km, a couple hundred J of CAPE, and frontal passage. Mid-level lapse rates are okay - roughly 6 C/km. Most models are in agreement for a decent amount of CIN, which would inhibit instability. Best chances for isolated thunderstorm development should be confined to the west and northeast. Prolonged southerly flow off the Gulf of Maine is anticipated to keep the Downeast region stable. Roughly anticipating about a quarter of an inch to half an inch of rain with these showers. Could be some locally higher spots of higher rain totals in regions favoring thunderstorm development.

AVIATION /13Z FRIDAY THROUGH TUESDAY/

Today: MVFR/IFR for Aroostook terminals with showers. PROB30 for CAR, PQI and HUL for tstms and vsbys between 15-18z. E-SE winds will increase to around 10 kts, with gusts picking up to 20 kts by midday.

KBGR, KBHB: VFR/MVFR today. Winds will become SE at 5 to 10 kts. Included a PROB30 for afternoon thunderstorms (-TSRA) at KBGR between 16z and 21z.

Tonight: Poor flying conditions expected region-wide. Widespread IFR to LIFR conditions will settle in across all terms through the end of the TAF period as -SHRA/BR, and very low stratus prevail. Cigs to drop below 500 feet (OVC002-OVC005) at several sites, including KFVE, KBGR, and KBHB. Winds will generally remain E-SE at around 10 kts across the north, and SE around 5 kts Downeast.

Saturday...IFR/LIFR with FG/BR in the AM becoming VFR/MVFR late morning. VFR by afternoon. SE winds shifting W-NW 5-15kt.

Saturday Night...VFR at KHUL/KBGR/KBHB. Chance for MVFR and isolated VCTS at KFVE/KCAR/KPQI. Winds light and variable.

Sunday-Sunday Night...MVFR, or lower, due to rain showers and potential thunderstorms in the morning and day. S winds with gusts 15-25KT possible Sunday. LLWS possible Sunday night. Monday...MVFR in rain showers in the morning. Potential for isolated VCTS at KFVE and KCAR Monday evening. S winds, shifting W/SW by afternoon, with gusts 15-20KT possible.

Monday night-Tuesday...VFR. W winds, 5-10KT. Chance for 20KT gusts Tuesday afternoon.

MARINE

Winds/seas below SCA conditions through Saturday for the coastal waters out 25nm. Fog will continue the entire time across all the waters out to 60nm reducing vsby at times less than 1nm. Showers are likely tonight into Saturday AM. Winds and seas below SCA criteria Saturday night into Sunday. Chance for a few gusts to 25 kts, and some 5 ft seas, over outer waters Sunday night into early morning. Fog over the waters Saturday night into Sunday morning. Rain showers move in Sunday night, and a chance for thunderstorms on Monday.

CAR WATCHES/WARNINGS/ADVISORIES

ME...Flood Watch from 2 PM EDT this afternoon through late tonight for MEZ001-003-004-010-031. MARINE...None.