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Strong-severe thunderstorms possible this afternoon & this evening over northern Maine (May 4, 2010)

Today and Tonight:  Satellite imagery and surface analysis showed a strong weather disturbance over Ontario province that is tracking east-northeast.  This weather disturbance will be the trigger for showers and thunderstorms this afternoon into this evening. 

I am expecting showers and thunderstorms this afternoon into this evening as a frontal boundary sets up across northern Maine.  The severe weather indices this afternoon into this evening are as follows: CAPE values: 500 to 650 j/kg,  lifted indexes: minus 1 to minus 2, Total total index: 55.  In addition, shear values of around 30 knots and helicity values of 150 to 190 will cause some rotation in the thunderstorms.  Also, the 850 to 500 millibar lapse rates will exceed 7 degrees Celsius per kilometer this afternoon into this evening aiding in the instability. 

The forecast sounding profiles look moist with a v-notch showing up below 5000 feet.  Why is this important??  V notch signatures are commonly associated with very strong updrafts and are commonly associated with supercell thunderstorms. 

Based on all of this information, I am going with showers and thunderstorms this afternoon into this evening.  Some of these thunderstorms will contain strong winds.  In addition, hail seems very possible with any thunderstorms as well.  The timing of the showers and thunderstorms are described in the paragraph below.   

One additional tidbit of information, the 4 km NAM model forecast radar forecast is pointing to thunderstorms entering western Aroostook County by 3 to 4 pm and making it into eastern Aroostook County, including Caribou, Presque Isle and Houlton between 5 and 7 pm.  Other model guidance has shower and thunderstorm activity starting as early as 12 to 1 pm.  Will take an average of the two and go with showers and thunderstorms from about 2-3 pm this afternoon to about 11 pm this evening. 

Thunderstorms will push into New Brunswick after 11 pm this evening and I expect variable clouds after midnight. 

High temperatures today will be near 70 Degrees.  Low temperatures tonight will be near 45 Degrees.

Wednesday through Thursday:  A trough of low pressure will hang back across the region Wednesday and will keep the threat for scattered showers across northern Maine on Wednesday.  High temperatures Wednesday will be near 58 Degrees. 

A frontal system will cross the region Thursday.  I expect scattered showers Thursday with the chance for afternoon thunderstorms as the atmosphere will become somewhat unstable.  I am not expecting strong or severe thunderstorms on Thursday afternoon.  High temperatures Thursday will be near 62 Degrees. 

Friday through Sunday:  Main item of interest will be the weekend storm system and when exactly will it rain.  Looking at the model guidance, the GFS model by midday Saturday and continuing through all day Sunday.  The Canadian model keeps us dry all day Saturday and brings the rain in Saturday night and continues it through Sunday.  An extrapolation of the European model points to rain developing by mid to late afternoon Saturday and continuing through Sunday. 

I will base the weekend forecast on an average of the three models above and go with rain developing by mid-afternoon Saturday and continuing through Saturday night and all day Sunday. 

The Forecast

Today:  Sunny this morning, then scattered showers and thunderstorms after 2 pm this afternoon.  Some thunderstorms may be strong to severe with strong winds, hail and frequent lightning.  Highs near 70 Degrees.  Winds West 10 to 15 mph.

Tonight:  Scattered showers and thunderstorms this evening, ending around 11 pm, then variable clouds after midnight.  Some thunderstorms this evening may be strong to severe with strong winds, hail and frequent lightning.  Lows near 45 Degrees.  Winds Northwest 5 to 10 mph.

Wednesday:  Mostly cloudy with scattered showers in the morning, then a mix of sun and clouds with scattered showers in the afternoon.  Highs near 58 Degrees.  Winds Northwest around 5 mph.

Wednesday night:  Partly cloudy.  Lows near 48 Degrees.  Winds South 5 to 10 mph.

Thursday:  Mostly cloudy with scattered showers in the morning, then mostly cloudy with scattered showers and a chance of thunderstorms in the afternoon.  Highs near 62 Degrees.  Winds South 5 to 10 mph in the morning and Southwest 10 to 20 mph in the afternoon.

Thursday night:  Partly cloudy.  Lows near 38 Degrees.   

Friday:  Partly sunny.  Highs near 58 Degrees.  

Friday night:  Mostly cloudy.  Lows near 35 Degrees. 

Saturday:  Cloudy in the morning into the early afternoon hours, then rain during the mid and late afternoon hours.  Highs near 55 Degrees.

Saturday night:  Periods of rain.  Lows near 40 Degrees.  

Sunday:  Periods of rain.  Highs near 51 Degrees.   

Prepared by Rob and Meg Lightbown,  Crown Weather Services

Disclaimer:  All forecasts herein are made to the best ability of the forecaster.  However, due to standard forecasting error, these forecasts cannot be guaranteed. Any action or inaction taken by users of this forecast is the sole responsibility of that user.

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