Ward Boston

Synopsis:

Possible near record breaking temperatures today throughout the forecast area as a high pressure system off the coast and to the south pulls in warm air. As a relatively strong low pressure system over the upper peninsula Michigan, moves off to the northeast into Canada, a cold front will move into the area Tuesday morning. This front will not bring precipitation but will bring a slight drop to the temperatures.

Current:

A Red Flag Warning is currently in place for much of Massachusetts. Current surface observations have a temperature of 79°F for Boston, and a record breaking temperature has been recorded in Worcester of 86°F at 12:25 pm EDT. Winds are relatively calm at the moment from the south, but should increase as the day progresses further enhancing the fire threat. A relatively dry air mass is in place over the forecast area with dewpoint temperatures in the low 50's right now. At the Boston area, a 5 knot sea breeze is currently in place, bringing in cooler temperatures off the Atlantic, but the more dominant southerly flow should take over as the day progresses. Current satellite imagery shows the frontal zone over the Ohio River Valley tracking eastward with most clear skies in place for the MA area, while current water vapor imagery supports the dry air mass currently moving into the area. High temperature today expected to be in the low 80's once the sea breeze shuts down.

Surface Obs at 1807Z, note the sea breeze in place over Boston.

Water Vapor Imagery at 1815 UTC.

Short Range (Monday April 16 - Thursday April 20):

Throughout this evening and tonight, a low pressure system will track off to the north bringing with it a cold front in tow. According to the 12z run of the NAM, an analysis of surface winds and theta-e shows the front passing through around 7 UTC Tuesday. Very minimal moisture in the lower levels with the passage of this front has eliminated any chance for precipitation, but some cloudiness is possible with the upward lifting associated with the front.

Surface Theta-E, 10 m winds (knots), and MSLP at 6Z April 17 showing the front ahead of the Boston area. twisterdata.com

925 mb Relative Humidity at 6Z Tuesday showing values of 50% at the timing of the frontal passage.

700 mb Relative Humidity for the same forecast hour showing even lower moisture.

A high pressure system off to the west and behind this front will gradually move into the area Tuesday morning and model soundings are showing an inversion layer taking hold and further increasing stability thus greatly reducing the chance of precipitation. Subsidence from this high pressure system will keep the air mass dry and dewpoint temperatures low throughout the short range forecast. The high pressure system could also bring in cold air advection aloft to help keep the high on Tuesday down, and combined with a tightening pressure gradient stronger winds are possible, especially with the 850mb jet streak.

By Wednesday, zonal flow at the 500 mb level will take hold and remain in place through Thursday when the NAM hints at a trough starting to dig in out west, and models are fairly consistent with the GEFS in good agreement. This will keep high temperatures and wind speeds down (edit: I think I forgot half of my sentence but I can't remember what "this" was). NAM shows a little shortwave to the south over Maryland, but most of the precipitation should remain well to the south in association with this forcing at around 3Z Thursday

Long Range (Thursday-Saturday):

Friday 0Z, the GFS shows a deepening trough over the Great Plains. Favorable jet position and strong southerly flow advecting in moisture could keep precipitation in the forecast, but a weak ridge over the Atlantic will keep the chance for POPs low. GFS favors strengthening the ridge in the Atlantic, and the GEFS starts showing uncertainty for the timing of the next low pressure system for Saturday, thus keeping a minimal chance for precipitation in the forecast for this weekend.

Verification:

A record high was observed in Boston of 87F with a low of 59F on the 16th. Winds finally did swing around from the sea breeze to a more southerly flow from the S SW with the max wind speed being 21 mph. Tuesday, Boston saw a high 84F and a low of 57F and was partly cloudy most of the day with winds from the west at 23 mph. The major cool down happened on Wednesday as the high was only 62 with a low of 49, as mostly cloudy skies were dominant throughout the day with winds from the NW at 20 mph. Based on surface observations, primarily the wind shift, the cold front passage was significantly slower than what the NAM was suggesting, not passing into Boston until 10Z on the 17th.

The sea breeze on the 16th appeared to last from 14Z to 1830Z before finally transitioning over to the strong southerly flow which was when the temperature hit the record maximum.

Observed soundings from Chatham, MA supports the NAM model soundings. The 0Z observed sounding showed the strong inversion layer, but the model tended to be a bit warmer than either the 0Z or 12Z sounding for the 17th. The higher wind speeds were forecasted fairly well for Tuesday, the biggest issue though was the timing of this front, and how much of a temperature change would be associated with the front. This was especially true, when heavier cloud coverage was forecasted, but IR satellite imagery showed clearing after the frontal passage, so the temperatures were able to reach back up into the mid 80s with daytime heating. Possibly the urban heat island effect, or the timing of the front kept Boston significantly warmer than areas further west for Tuesday where most areas were just under 80F.

A heavy cloud deck established itself on Wednesday, helping to keep the temperatures down. The NAM 0Z run on the 17th showed a 500 mb vort max swing into the area which may have aided in the enhanced cloud coverage by providing sufficient lift of any moisture present. Currently, for the 19th, a sea breeze is in place, but forecasting a sea breeze 4 days out is not practical.