textproduct: Gray - Portland

WHAT HAS CHANGED

Quick update to cancel the Winter Weather Advisory a little bit early. While some of the highest peaks could still see some patchy freezing rain, road temperatures have generally warmed above freezing.

KEY MESSAGES

1. A brief bout of light freezing rain early this morning may bring about slippery travel across the north and down into the Monadnock region through the morning commute. Impacts are mainly expected at higher elevations.

2. A potent low pressure system will bring heavy rain and gusty winds to most of the forecast area today through Tuesday morning. Have expanded the wind advisory to some zones primarily for this evening. Some localized flooding is possible as a result of significant runoff and snowmelt. The risk of flooding as a result of ice jams remains low, but non- zero as some ice does remain in the mountains and northern basins.

3. Colder air briefly returns midweek with quieter weather before temperatures trend back toward normal for the second half of the week.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION... Temperatures are expected to bottom out below freezing in the north, and in the higher elevations of the white mountains and Monadnock region. Weak cold air damming likely allows a period of light freezing rain to occur this morning, before the warm front sweeps through and changes it over to plain rain by the afternoon. Amounts look light, a tenth of an inch or less, and should be mainly confined to the higher elevations. A Winter Weather Advisory is in effect from 3 AM to noon for potentially slick travel.

KEY MESSAGE 2 DESCRIPTION... Not much has changed with regards to timing as precipitation is expected to start moving into the area around 3 to 4 am. As mentioned above it may start as a brief period of freezing rain for the higher elevations, but for most it will be plain rain. It starts light, but with anomalous PWATs, and CAMs continuing to suggest a couple hundred joules of MUCAPE present, it won't take long for heavier rain to develop. Impacts with this first round likely come in the form of ponding on roadways and low visibilities as models continue to target the I-95 corridor and the Midcoast with higher rainfall totals. There remains good agreement on a lull of sorts for a good part of the afternoon as activity becomes more showery, but forecast soundings suggest any break in rain will be filled with fog and/or drizzle as moisture transport into the region increases. The second round of rain arrives Monday evening and crosses through the overnight period as a cold front swings through. As with the first round, instability and anomalous PWATs (that will be even higher at this point) will allow for it to be heavy at times. WPC has put us in a Marginal ERO, meaning the flash flood risk is non-zero, however the progressive nature of these rounds of rains should help to keep this risk very low. That being said don't be surprised if you see some nuisance flooding in urban and low lying areas. Runoff from excessive rainfall in the mountains poses a minor riverine flooding risk along with warm temperatures allowing for any remaining ice to shift and move as well. For more details on these risks see the hydrology section below. Confidence in rainfall totals around 1-2 inches remains high, with confidence increasing in locally higher totals as high as 3" on the Midcoast and in the White Mountains.

Another concerning aspect of this system is the winds, mainly because of how much uncertainty still surrounds them. During the morning hours 850 mb winds increase to 50-60 knots with forecast soundings continuing to suggest a strong inversion preventing efficient mixing. However, the pressure gradient alone should be good enough for us to see gusts 20-25 mph during the day. The more concerning period is the evening as the low level jet increases. Some solutions are now suggesting winds as high as 100 knots just off the deck over the coastal plain. Again, forecast soundings hold on to the inversion but the concern comes in those times where enhanced precipitation may allow for higher gusts to mix down, with the question being how much momentum will be behind them. The Wind Advisory highlights the areas of highest confidence for frequent 40-50 mph gusts, but the Midcoast could see gusts 55-60+ with the push Monday evening. Confidence isn't high enough to upgrade to a High Wind Warning anywhere at the moment, and if it is going to occur with convective activity we could handle it with short fuse warnings as well. The other area to watch will be on the north sides of the mountains as downslope enhancement of the already strong winds is possible. The power outage threat is limited across most of the area but is locally elevated within the Wind Advisory. Lastly, a quick note on coastal flooding as I am sure the strong onshore flow has brought this to some folks minds. This is not a primary concern with this system. The astronomical tide is only around 9 feet at both of Monday's high tide cycles and it looks like the peak of the winds is only going to be occuring at the lower of the two which is Monday night. Surge models are currently forecasting around a foot and change of surge which would not even bring us to action, and therefore no products seem necessary at this time. As always, we will keep an eye on it just in case.

Precipitation begins tapering off Tuesday morning with cooler air quickly filling in behind the front. High temperatures top out in the mid to upper 30s north of the mountains and in the 40s to the south. A tight pressure gradient and efficient mixing is going to allow for west/southwesterly gusts 30-35 mph to continue through the day which will make it feel a bit colder.

KEY MESSAGE 3 DESCRIPTION... A period of more tranquil but cold weather is expected after the departure of the aforementioned system. Northwesterly winds and very cold temperatures will allow for a bitterly cold morning on Wednesday. Wind chills are expected to be in the single digits below zero in the mountains, with single-digits above elsewhere.

High pressure continues through the rest of the week, with temperatures gradually warming as the mid-level trough makes a slight realignment to the west. The next chance for precipitation looks to be next weekend, though models remain very uncertain on timing and storm track.

AVIATION /14Z MONDAY THROUGH FRIDAY/

Through 12Z Tuesday...IFR conditions will continue through much of the day under low ceilings and periodic heavy rain. Scattered LIFR conditions will be possible through the day as well but there could be some breaks in precipitation this afternoon. Enhanced low- level wind shear around 35-40 kts will also be likely at almost all sites starting late this evening. High confidence in widespread IFR/LIFR cigs continuing as a second round of rain arrives after 00Z Tuesday. LLWS continues with many terminals seeing an increase to 45-50kts, with strong SE to S surface gusts, 30-40 kts, along at least coastal terminals.

Outlook...

Overnight Monday-Tuesday: Precipitation will begin to taper off early Tuesday morning with conditions slowly improving to VFR at most terminals by Tuesday afternoon. Gusty winds 30-35 kts will persist through the day.

Wedenesday-Friday: VFR expected, with a slight chance of lower CIGs at HIE due to a stray snow shower on Friday.

Saturday: VFR expected, though restrictions can not be ruled out for light rain and snow.

MARINE

Onshore flow will increase through the day with frequent gales likely by early this morning and frequent storm force gusts likely by tonight. Storm force gusts taper off early Tuesday morning with gales tapering off by Tuesday afternoon. Winds gusts 25-30 kts continue through Tuesday evening. This wind will build wave heights to a peak of 12-15ft early Tuesday morning, and then they will very slowly start to fall through the day Tuesday. This strong system will bring heavy rain an possibly thunder to the waters through tonight.

Seas of 8-12ft and westerly gales are expected Tuesday night. Conditions start to improve by Wednesday morning, with sub-SCA level westerlies and 6-10ft seas expected by then. Sub-SCA winds and seas of 3-7ft are expected through the remainder of the week.

HYDROLOGY

A Flood Watch has been issued for today into Tuesday as an anomalously moist airmass interacts with the ripe snowpack across the area. The main focus is on excessive runoff when the heaviest rainfall tonight coincides with rapid melt, leading to dramatic rises in area streams and rivers. There is high confidence that we will see 1 to 2 inches of rain, with some ensembles targeting the White Mountains and the Midcoast of Maine with over 3 inches. Snowmelt will be limited initially due to cold air damming in the mountains, but this will erode late this morning as moisture surges north under a powerful 70 to 90+ knot low- level jet. Given the anamolous moisture, expected PWATs should be in the 1.2 to 1.4 inch range (near the 99th percentile for mid- March). Thus, the environment should be primed for heavy rainfall. The late moisture surge tonight will be the real nudge needed to push water out of the already ripened snowpack. The expected snowmelt water loss of 1 to 1.5 inches will mostly occur between this evening and early Tuesday morning, about 6 to 12 hours. The coinciding of heaviest rainfall with rapid snowmelt will lead to swift rises on small streams and rivers. The mainstream rivers have a lot of channel capacity due to lingering drought, but more confined channels in the mountains or small streams could easily be overwhelmed resulting in localized minor flooding. Deep frost will limit infiltration, so for some areas the sheer volume of water could overwhelm culverts, low-lying areas, and low-water crossings. Expect urban street flooding and drainage issues. While many southern rivers are ice-free, some ice remains in the mountains and northern basins. There remains a non-zero risk for mechanical break-up and unpredictable jams as flows increase.

GYX WATCHES/WARNINGS/ADVISORIES

ME...Flood Watch from 2 PM EDT this afternoon through Tuesday afternoon for MEZ007>009-012>014-018>028-033. Wind Advisory from 8 PM this evening to 8 AM EDT Tuesday for MEZ014-021>028. Wind Advisory from 8 PM this evening to 5 AM EDT Tuesday for MEZ018>020. NH...Flood Watch from 2 PM EDT this afternoon through Tuesday afternoon for NHZ001>015. Wind Advisory from 8 PM this evening to 8 AM EDT Tuesday for NHZ014. Wind Advisory from 8 PM this evening to 5 AM EDT Tuesday for NHZ007>013-015. MARINE...Storm Warning from 8 PM this evening to 8 AM EDT Tuesday for ANZ150>152-154. Gale Warning until 8 AM EDT Tuesday for ANZ153.