Mt d'Arbois - 29/07/21

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Mt Blanc Valley Weather - 3 Day Forecast

TENACIOUS SOUTHWEST Aores High Ridge is currently withdrawing in the Bay of Biscaye, while 1000hpa low keeps deepening east close to Scotland, more and more interacting with the large altitude cut-off whirling around Northwestern Ireland, and which extends an altitude cold air thalweg up to Southern France. Resulting cold front, stretching Baltic Sea to Gulf of Lions, triggers thunderstorm while crossing over our High Massifs this morning, bound towards Italy. After a temporary pressure rise which will cause a fair sunny lull by Thursday, the tenacious southwest pattern should feature several supplemental peaks of humidity and instability: during the night to Friday, Friday evening, Saturday evening, and may be again on Sunday all over the day. Thuderstrom risk must be quite considerable by then.
TODAY - WEDNESDAY JULY 28
Thundery shower, then improving later in the morning - fair sunny spell spreading later
SKY CONDITION : overcast or mostly cloudy until 11:00 or so, ceiling often below 2000m - improving quite suddely thereafter, fair sunny spell spreading out - then convection expected to resuming, triggering locally bulky cumulus cloud growth during the afternoon. Period of sunshine close to 35%.
PRECIPITATION : thundery shower, locally fairly heavy until 10:00, becoming light and scattered, then ending before noon - rain-snow limit towards 3200m - amount of water in the valley during last 3hours: 8-12mm.
WIND - ground level : calm -> SW light.
WIND - mid mountain : SW moderate to strong -> WSW light to moderate.
WIND - high mountain : SW moderate -> W moderate to strong.
TEMPERATURE : low +14C - high +24C.
ISOTHERM 0C : 3600 -> 4000 m.
ISOTHERM -10C : 5300 -> 5700 m.
OVERNIGHT
Fair summer night, clear moonshine - quite cool in the valley by dawn
SKY CONDITION : clear on the whole - excellent radiation condition providing snow surface re-freezing highup -iso 0C dropping towards 4200m.
PRECIPITATION : none.
WIND - aloft : W moderate to strong -> moderate.
TEMPERATURE : low +10C.
THURSDAY JULY 29
Fair summer weather - somewhat sultry over the afternoon hours - windy highup
SKY CONDITION : clear in the morning - becoming partly cloudy in the afternoon, some high and median altitude cloud running over from southwest - locally cloudy due to cumulus cloud growth. Period of sunshine close to 80%.
PRECIPITATION : none before nightfall.
WIND - aloft : W moderate -> WSW moderate to strong.
TEMPERATURE : low +9 C - high +27C.
FRIDAY JULY 30
Uncertain - resuming instability - morning sunny spells, then showers - windy high up
SKY CONDITION : partly cloudy in the morning - becoming cloudy, or even locally mostly cloudy in the afternoon due to cumulus cloud growth. Period of sunshine close to 40%.
PRECIPITATION : thundery shower likely in the afternoon - rain-snow limit towards 3600m.
WIND - aloft : WSW moderate to strong.
TEMPERATURE : rising lows - dropping highs.
EXTENDED OUTLOOK UNTIL MONDAY AUGUST 2 2021

Saturday: cloudy with fair sunny spells - shower risk seems very low - west-southwest highup, fairly strong, probably decreasing later. Sunday: uncertain - often mainly cloudy - some sunny spells in the morning - afternoon shower. Monday: very uncertain - fairly sunny in the morning - cloudy in the afternoon, owing to convection, probably without precipitation.

Forecast reliability : fairly good until Thursday - poor thereafter.