G-550 captain reported a TCAS RA and near midair collision with a PA28 in marginal VFR conditions.

Attributes
ACN	1426532
Time
Date	201702
Local Time Of Day	1201-1800
Place
Locale Reference	ZZZ.Airport
State Reference	US
Environment
Flight Conditions	Marginal
Light	Daylight
Aircraft 1
Make Model Name	Gulfstream V / G500 / G550
Operating Under FAR Part	Part 91
Flight Phase	Initial Climb
Flight Plan	IFR
Aircraft 2
Make Model Name	PA-28 Cherokee/Archer/Dakota/Pillan/Warrior
Operating Under FAR Part	Part 91
Flight Phase	Initial Approach
Person 1
Function	Captain Pilot Not Flying
Qualification	Flight Crew Air Transport Pilot (ATP) Flight Crew Flight Engineer Flight Crew Instrument Flight Crew Multiengine
Experience	Flight Crew Last 90 Days 100 Flight Crew Total 11000 Flight Crew Type 500
Events
Anomaly	Conflict NMAC Inflight Event / Encounter Weather / Turbulence
Miss Distance	Horizontal 300 Vertical 100

Narrative:

After leveling on the departure procedure at 1;700 feet MSL; we turned to a heading of 113 and initiated a climb per the departure procedure. Immediately after climb initiation we simultaneously received a TA then immediately switching to a level RA and also a traffic advisory from tower. There was a VFR target at 12 o'clock and approximately a mile +300 feet (~2;100 MSL) that was initially a TA then a RA. After initiating a level off the TCAS switched to a climb RA which was then followed.we were not warned by tower until the beginning of the TA/RA event. I visually picked up the PA28 in a steep left hand turn at 1/2 mile and 1 o'clock. The aircraft passed approximately 300 feet off the right wing. After passing the traffic at 2;100 feet we were immediately IMC. There was no possibility of the PA28 being 500 feet below the overcast base as VFR rules dictate. I did call the tower for feedback after landing at destination; but was informed the supervisor was investigating and would return my call in the future. The VFR traffic was inbound from the south and was attempting to enter a left downwind I would guess. We were then transferred to departure control and the remainder of the flight was uneventful. It was difficult to visually acquire the PA28 due to rain and low ceilings.

Original NASA ASRS Text

Title: G-550 Captain reported a TCAS RA and NMAC with a PA28 in marginal VFR conditions.

Narrative: After leveling on the departure procedure at 1;700 feet MSL; we turned to a heading of 113 and initiated a climb per the departure procedure. Immediately after climb initiation we simultaneously received a TA then immediately switching to a level RA and also a traffic advisory from Tower. There was a VFR target at 12 o'clock and approximately a mile +300 feet (~2;100 MSL) that was initially a TA then a RA. After initiating a level off the TCAS switched to a climb RA which was then followed.We were not warned by Tower until the beginning of the TA/RA event. I visually picked up the PA28 in a steep left hand turn at 1/2 mile and 1 o'clock. The aircraft passed approximately 300 feet off the right wing. After passing the traffic at 2;100 feet we were immediately IMC. There was no possibility of the PA28 being 500 feet below the overcast base as VFR rules dictate. I did call the Tower for feedback after landing at destination; but was informed the supervisor was investigating and would return my call in the future. The VFR traffic was inbound from the south and was attempting to enter a left downwind I would guess. We were then transferred to departure control and the remainder of the flight was uneventful. It was difficult to visually acquire the PA28 due to rain and low ceilings.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.