Winter Storm Nova Event Summary

Winter/Spring Flood Potential Outlook For Northern And Eastern Maine - 02/16/2023

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WINTER/SPRING FLOOD POTENTIAL OUTLOOK

NATIONAL WEATHER SERVICE CARIBOU ME

1039 AM EST Thu Feb 16 2023

...SPRING FLOOD POTENTIAL OUTLOOK FOR NORTHERN, CENTRAL AND

DOWNEAST MAINE...

This is the fourth Winter/Spring Flood Potential Outlook for

2023, issued by the National Weather Service in Caribou, Maine.

This outlook is for Northern, Central, and Downeast Maine for the

two- week period of February 16th to March 2nd, 2023.

The potential for ice jam flooding is normal across Northern &

Central basins. The potential for ice jam flooding is below normal

across Southern basins. The potential for open water flooding at

all basins over the next two weeks is below normal.

...CLIMATOLOGICAL GUIDANCE...

Apart from a short-lived...intense Arctic outbreak in early

February the overall pattern of warmer than normal temperatures

from January has continued. Looking back on January 2023 it was

much warmer than average compared to the 1991-2020 climate

normals. It featured long stretches of above to well above average

temperatures and only brief periods of below average

temperatures. This was a result of the monthly average of the

North Atlantic Oscillation (NAO) pattern monthly mean of +1.25

standard deviation, while the Pacific North American (PNA) pattern

at +0.21 standard deviation. This typically results in higher

500mb heights over the Northeastern United States and typically a

progressive jet stream flow from west to east with brief deeper

troughing over the east. We saw anomalous higher heights for the

month over the area with nearly no blocking in the North Atlantic

resulting in generally high pressure over the North-Central

Atlantic west of Spain.

Temperatures across the area ranged from 6.5 to 9.5 degrees above

the 1991-2020 climate normals. The month only had 6 days below

average temperatures at Caribou and only 4 days below average

temperatures at Bangor. It ranked as the 4th warmest January on

record at Caribou, and the 2nd warmest at Houlton, Millinocket and

Bangor. Bangor only had one night the entire month with a low

temperature below zero and the lowest low temperature for the

month was 2 below zero. On average, Bangor observes 8 nights with

a low temperature below zero in January. At Caribou, there were

only 4 nights with a low below zero; and typically there is an

average of 15 nights with sub-zero lows. For February,

temperatures are running overall around to slightly above normal

across the Crown with most other locations 1.5 to 2.5 degrees

above normal.

Precipitation for the month ranged from 150 to 200 percent of

average across Eastern and Northern Maine. A total of 5.33 inches

of rain and melted snow & sleet was observed at Caribou which was

the 3rd highest total on record for the month of January. A total

of 5.82 inches of rain and melted snow & sleet was observed at

Bangor which is the 4th highest total on record. Liquid

precipitation (Rain, Melted Snow, Sleet and Freezing Rain) is

running above normal across the entire area for the month of

February.

The NAO Index has been positive for virtually all of January and

early February. With a positive NAO...there tends to be a very

fast zonal flow across the northern United States and southern

Canada. Any cold air in Canada is quickly shunted east into the

North Atlantic Ocean in the fast zonal flow. So...Arctic

intrusions...although not impossible...tend to be short lived and

quickly moderate. This is exactly the scenario we saw earlier this

month. Very cold temperatures were quickly replaced with much

above normal temperatures in roughly a 48-hour period. There are

some signs that the NAO may weaken or perhaps try and turn

negative as we near the end of February or early March. If this

happens...temperatures would likely turn much more seasonal and

there would be a better chance to see increased snowfall.

The PNA pattern has been neutral to slightly positive so far this

February. There are signs that the PNA index will be turning a

bit more negative as we head into the end of February. With a

negative PNA...upper level ridging is favored across the southeast

United States which typically results in more of a broad

southwest flow across the east coast and warmer than normal

temperatures in at least southern portions of the CWA. The

combination of a positive NAO and negative PNA is typically not

very conducive to snow across much of the area. However, medium

range numerical weather forecast models suggest the cold air may

become just entrenched enough for mainly wintry precipitation

during the last week or so of February especially across northern

Maine.

The official Climate Prediction Center (CPC) medium range

forecasts seem to agree with this scenario. Temperatures are

expected to average above normal from 20 to 24 February 2023

across all of New England. However, that trend flips during the

last week or so of February when temperatures are likely to

average below normal for all areas. Precipitation is expected to

be above normal region wide from now through the end of February.

...OBSERVED SNOW DEPTH AND WATER EQUIVALENTS...

The Maine Cooperative Snow Survey fourth survey was this week

showing a rather impressive snowpack across Northern Maine with a

below normal pack in southern areas of our forecast area. Coastal

areas from Bangor to Bar Harbor to Calais were generally reporting

5 to 10 inch snow depths with several areas at the shoreline at a

trace. Snow depths were zero to a trace at Machias. Snow water

equivalents mostly vary from 0.00 to 3.00 inches in this region.

Snow depths quickly increase nearing the Central Highlands to

Moosehead Region including Interior Downeast with 12 to 18 inches

and snow water equivalents range from 3.00 to 5.00 inches with

increased water loading of the snowpack in this region. Heading

across northern Maine, snow depths generally range from 20 to 30

inches with some higher depths up to 35 to 40 inches northwest of

Caribou into the Saint John Valley. Higher terrain areas were

observing 2 to 4 foot snow depths. Chimney Pond at Katahdin

recently reported a 48 inch snow depth...an increase of 1 inch

since the last outlook. Snow water equivalents generally range

4.00 to 8.00 inches locally higher across the peaks. The highest

snow water equivalents were found along the Big Black River at

Clayton Lake...Allagash River...Fish River and Saint John River at

Fort Kent...Aroostook River at Washburn.

...SOIL MOISTURE AND WATER SUPPLY CONDITIONS...

Frozen grounds generally 2-8 inches across the area with deeper

frost in southern areas due to less snowpack. Frozen grounds

combined with no melt has allowed for virtually no changes in soil

moisture conditions. Near surface soil moisture anomalies show

all of Eastern & Northern Maine having above to well above normal

soil moisture. The latest Palmer Drought Severity Index Map from

February 9, 2023 shows the Downeast Abnormally Wet, Greater Bangor

is Extremely Wet and much of the remaining areas are Severely to

Extremely Wet. There is below normal to well below normal soil

moisture across the far North Woods along the Quebec border. The

Palmer Index looks at conditions over the span of weeks to months.

Taking a look at groundwater monitoring wells across the region courtesy

of the United States Geological Survey (USGS), we see much of the

groundwater level observing sites are well above normal to

extremely high for early February. All reporting sites are Above

Normal in the >75th percentile range with most reporting Much

Above Normal (>90th percentile) and Calais reporting historic

levels, however there is only 23 years of data there.

Lastly, given the slow discharge of the rivers we are looking at

still very high river storages. We focus on the observations we

get for the Penobscot River basin. The Penobscot total storage as

of February 13, 2023 was 67.6% full which is 36.4% above the long

term average.

...RIVER AND ICE CONDITIONS...

As of February 15th nearly every river, stream and creek with

USGS gages in Eastern & Northern Maine are reporting being

impacted by ice. Overall, natural flows are running normal to

above normal across the entire area.

Along the Saint John headwaters there is a mix of hummocked ice

and thick sheet ice approximately 8-14 inches in spots.

Significant hummock ice was found between Allagash to the Saint

Francis to Fort Kent town line. There are some very small openings

in the narrow gauge area in Saint Francis. Ice thickness

estimates were generally 8 to 11 inches with locally thicker

chunks. There has been snowmobile activity on the river along with

moose using the ice to get across between each bankside. In Fort

Kent through to before Madawaska there is some opening but with

cold air a significant increase in ice is noted. Significant

hummock ice stretches from Madawaska to Van Buren and then

downstream to Hamlin. There are some very small openings in

Madawaska around the construction site of the new International

Bridge at the port of entry.

Along the Allagash river there is a mix of hummock ice and

thickening sheet ice with more significant ice from the Allagash

River gage to the confluence with the Saint John River. Ice

thicknesses here were ranging from 4 to 10 inches with locally

thicker ice but due to significant snowpack on the ice these are

just estimates based on wildlife activity and recreational uses.

The Aroostook river is frozen between Millinocket Lake to

Masardis with a few small leads, it is noted snowmobiles are

crossing and using the river ice to travel from Oxbow Road

indicating much thicker ice. However, upstream from the Oxbow Road

it was noted several open leads indicating extreme variations in

ice thickness. Ice thicknesses near Masardis were 5 to 9 Inches

and the USGS survey team noted the ice as 100 percent black ice.

The Aroostook River is filled with hummocked ice between the

confluence of the Scopan Stream through to Ashland before the

Sheridan Road R/R trestle. Between Wade and Presque Isle through

Washburn ice thicknesses are estimated 7 to 12 inches based solely

on wildlife activity and USGS survey team measurements. Hummocked

chunks 1-3 ft thick along banking`s and caught on islands after

water levels dropped. Presque Isle to Caribou dam features sheet

ice and large hummocked ice along the banking`s. The Caribou Dam

induced opening from the Dam shortly to the Fort Fairfield Road

bridge. Beyond the bridge downstream past the Little Madawaska

River confluence into Fort Fairfield to the New Brunswick border

the ice is very hummocked with some massive thicknesses. In the

main channel the river ice is thickening to 5-9 inches noted by

wildlife activity and occasional snowmobiles venturing out on the

ice along with human activity.

Along the Piscataquis River there are frozen sections along the

headwaters and downstream to the confluence with the Penobscot

River. Ice thickness is all over the place given the reservoir and

dam activities. However, ice thickness is thick enough to host

outdoor activities in some locations along the ice. The eastern

branch of the Penobscot River along with the North & West branches

all show several areas of hummock and thickening sheet ice. There

is significant frazil ice along with bank ice reported / noted

via USGS webcam in Bangor impacting the Penobscot and Kenduskeag.

The confluence of the Kenduskeag is mainly hummocked ice but some

sheet ice. The main channel of the Penobscot is mainly open water

with frazil in the flow. Downeast Rivers and streams have

significant ice development in this recent cold air including the

St. Croix between the dams where ice can form.

...IN CONCLUSION...

The potential for flooding due to ice jams is normal across

Northern Maine with above average flows currently occurring and

significant hummock ice there is a risk for flooding this spring

if ice jams develop. Given the above normal temperatures the ice

jam potential is now normal across the Central Highlands into

Downeast Maine. The open water flood potential is below normal

across the entire area.

The next Winter/Spring Flood Outlook will be issued on Thursday,

March 2nd.

$$

Sinko